Synthetic Δ17 desaturase derived from Phytophthora ramourum  and its use in making polyunsaturated fatty acids

ABSTRACT

The present invention relates to Δ17 desaturases, which have the ability to convert ω-6 fatty acids into their ω-3 counterparts (i.e., conversion of arachidonic acid [20:4, ARA] to eicosapentaenoic acid [20:5, EPA]). Isolated nucleic acid fragments and recombinant constructs comprising such fragments encoding Δ17 desaturases along with a method of making long chain polyunsaturated fatty acids (PUFAs) using these Δ17 desaturases in plants and oleaginous yeast is disclosed.

This application claims the benefit of U.S. Provisional Patent Application 60/793,575, filed Apr. 20, 2006.

FIELD OF THE INVENTION

This invention is in the field of biotechnology. More specifically, this invention pertains to the identification of nucleic acid fragments encoding Δ17 fatty acid desaturase enzymes and the use of these desaturases in making long chain polyunsaturated fatty acids (PUFAs).

BACKGROUND OF THE INVENTION

The importance of PUFAs is undisputed. For example, certain PUFAs are important biological components of healthy cells and are recognized as: “essential” fatty acids that cannot be synthesized de novo in mammals and instead must be obtained either in the diet or derived by further desaturation and elongation of linoleic acid (LA; 18:2 ω-6) or α-linolenic acid (ALA; 18:3 ω-3); constituents of plasma membranes of cells, where they may be found in such forms as phospholipids or triacylglycerols; necessary for proper development (particularly in the developing infant brain) and for tissue formation and repair; and, precursors to several biologically active eicosanoids of importance in mammals (e.g., prostacyclins, eicosanoids, leukotrienes, prostaglandins). Additionally, a high intake of long-chain ω-3 PUFAs produces cardiovascular protective effects (Dyerberg, J. et al., Amer. J. Clin. Nutr., 28:958-966 (1975); Dyerberg, J. et al., Lancet, 2(8081):117-119 (Jul. 15, 1978); Shimokawa, H., World Rev. Nutr. Diet, 88:100-108 (2001); von Schacky, C. and Dyerberg, J., World Rev. Nutr. Diet, 88:90-99 (2001)). And, numerous other studies document wide-ranging health benefits conferred by administration of ω-3 and/or ω-6 PUFAs against a variety of symptoms and diseases (e.g., asthma, psoriasis, eczema, diabetes, cancer).

A variety of different hosts including plants, algae, fungi and yeast are being investigated as means for commercial PUFA production. Genetic engineering has demonstrated that the natural abilities of some hosts (even those natively limited to LA and ALA fatty acid production) can be substantially altered to result in high level production of arachidonic acid (ARA; 20:4 ω-6), eicosapentaenoic acid (EPA; 20:5 ω-3) and docosahexaenoic acid (DHA; 22:6 ω-3). Whether ω-3/ω-6 PUFA production is the result of natural abilities or recombinant technology, both strategies may require conversion of ω-6 PUFAs into their ω-3 counterparts. Specifically, a Δ15 desaturase is responsible for the conversion of LA to ALA, while a Δ17 desaturase is responsible for the conversion of ARA to EPA (although some Δ17 desaturases can also use dihomo-γ-linolenic acid (DGLA; 20:3 ω-6) as a substrate to produce eicosatetraenoic acid (ETA; 20:4 ω-3)). Both of these enzymes have a role in the Δ6 desaturase/Δ6 elongase pathway (which is predominantly found in algae, mosses, fungi, nematodes and humans and which is characterized by the production of γ-linoleic acid (GLA; 18:3 ω-6) and/or stearidonic acid (STA; 18:4 ω-3)) and the Δ9 elongase/Δ8 desaturase pathway (which operates in some organisms, such as euglenoid species and which is characterized by the production of eicosadienoic acid (EDA; 20:2 ω-6) and/or eicosatrienoic acid (ETrA; 20:3 ω-3)) (FIG. 1).

Based on the role Δ17 desaturase enzymes play to thereby effectively enable the synthesis of ω-3 fatty acids, there has been considerable effort to identify and characterize these enzymes from various sources. However, only a few Δ17 desaturases are presently known. Specifically, U.S. Patent Publication No. 2003/0190733 describes the usoaltion and characterization of a Δ17 desaturase from Saprolegnia diclina (see also GenBank Accession No. AY373823). PCT Publication No. WO 2005/083053 provides sequences for a Phytophthora infestans “ω3 desaturase” (see also GenBank Accession No. CAJ30870), while PCT Publication No. WO 2006/100241 provides sequences for a Phytophthora sojae “ω3 desaturase”, both of which appear to encode Δ17 desaturases. Also, commonly owned, co-pending application having Provisional Application No. 60/855,177 (discloses amino acid and nucleic acid sequences for a Δ17 desaturase enzyme from Pythium aphamidermatum. Thus, there is need for the identification and isolation of additional genes encoding Δ17 desaturases that will be suitable for heterologous expression in a variety of host organisms for use in the production of ω-3 fatty acids.

Applicants have solved the stated problem by isolating the genes encoding Δ17 desaturase from the oomycetes, Phytophthora ramorum and Phytophthora sojae.

SUMMARY OF THE INVENTION

The present invention relates to new genetic constructs encoding polypeptides having Δ17 desaturase activity, and their use in plants and yeast for the production of PUFAs and particularly ω-3 fatty acids.

Accordingly the invention provides an isolated nucleic acid molecule selected from the group consisting of:

-   -   a.) an isolated nucleotide sequence encoding a Δ17 desaturase         enzyme, selected from the group consisting of SEQ ID NO:3 and         SEQ ID NO:7; or,     -   b.) an isolated nucleotide sequence that is completely         complementary to (a).

In another embodiment the invention provides genetic constructs of the invention that are codon optimized for expression in Yarrowia sp. Additionally the invention provides host cells transformed with the genetic constructs of the invention for expression and production of ω-3 fatty acids.

In another embodiment the invention provides a method for the production of eicosapentaenoic acid comprising:

-   -   a) providing a host cell comprising:         -   i) an isolated nucleotide molecule encoding a Δ17 desaturase             polypeptide, selected from the group consisting of:             -   1) an isolated nucleotide molecule encoding a Δ17                 desaturase polypeptide having at least 90.9% identity                 when compared to a polypeptide having the amino acid                 sequence as set forth in SEQ ID NO:2, based on the                 Clustal method of alignment;             -   2) an isolated nucleotide molecule encoding a Δ17                 desaturase polypeptide having at least 91.4% identity                 when compared to a polypeptide having the amino acid                 sequence as set forth in SEQ ID NO:4, based on the                 Clustal method of alignment; and,             -   3) an isolated nucleotide molecule encoding a Δ17                 desaturase polypeptide having at least 89.5% identity                 when compared to a polypeptide having an amino acid                 sequence as set forth in SEQ ID NO:6, based on the                 Clustal method of alignment; and         -   ii) a source of arachidonic acid;     -   b) growing the host cell of step (a) under conditions wherein         the nucleic acid molecule encoding the Δ17 desaturase         polypeptide is expressed and the arachidonic acid is converted         to eicosapentaenoic acid; and     -   c) optionally recovering the eicosapentaenoic acid of step (b).

In an alternate embodiment the invention provides a method for the production of eicosatetraenoic acid comprising

-   -   a) providing a host cell comprising:         -   i) an isolated nucleotide molecule encoding a Δ17 desaturase             polypeptide, selected from the group consisting of:             -   1) an isolated nucleotide molecule encoding a Δ17                 desaturase polypeptide having at least 90.9% identity                 when compared to a polypeptide having the amino acid                 sequence as set forth in SEQ ID NO:2, based on the                 Clustal method of alignment;             -   2) an isolated nucleotide molecule encoding a Δ17                 desaturase polypeptide having at least 91.4% identity                 when compared to a polypeptide having the amino acid                 sequence as set forth in SEQ ID NO:4, based on the                 Clustal method of alignment; and,             -   3) an isolated nucleotide molecule encoding a Δ17                 desaturase polypeptide having at least 89.5% identity                 when compared to a polypeptide having an amino acid                 sequence as set forth in SEQ ID NO:6, based on the                 Clustal method of alignment; and         -   ii) a source of dihomo-γ-linolenic acid;     -   b) growing the host cell of step (a) under conditions wherein         the nucleic acid molecule encoding the Δ17 desaturase         polypeptide is expressed and the dihomo-γ-linolenic acid is         converted to eicosatetraenoic acid; and     -   c) optionally recovering the eicosatetraenoic acid of step (b).

In another embodiment the invention provides a method for the production of α-linolenic acid comprising:

-   -   a) providing a host cell comprising:         -   i) an isolated nucleotide molecule encoding a bifunctional             Δ17 desaturase polypeptide, selected from the group             consisting of:             -   1) an isolated nucleotide molecule encoding a Δ17                 desaturase polypeptide having at least 90.9% identity                 when compared to a polypeptide having the amino acid                 sequence as set forth in SEQ ID NO:2, based on the                 Clustal method of alignment; and,             -   2) an isolated nucleotide molecule encoding a Δ17                 desaturase polypeptide having at least 91.4% identity                 when compared to a polypeptide having the amino acid                 sequence as set forth in SEQ ID NO:4, based on the                 Clustal method of alignment; and         -   ii) a source of linoleic acid;     -   b) growing the host cell of step (a) under conditions wherein         the nucleic acid molecule encoding the bifunctional Δ17         desaturase polypeptide is expressed and the linoleic acid is         converted to α-linolenic acid; and,     -   c) optionally recovering the α-linolenic acid of step (b).

BIOLOGICAL DEPOSITS

The following biological material has been deposited with the American Type Culture Collection (ATCC), 10801 University Boulevard, Manassas, Va. 20110-2209, and bears the following designation, accession number and date of deposit.

Biological Material Accession No. Date of Deposit Yarrowia lipolytica Y2047 ATCC PTA-7186 Oct. 26, 2005 The biological materials listed above were deposited under the terms of the Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purposes of Patent Procedure. The listed deposit will be maintained in the indicated international depository for at least 30 years and will be made available to the public upon the grant of a patent disclosing it. The availability of a deposit does not constitute a license to practice the subject invention in derogation of patent rights granted by government action.

BRIEF DESCRIPTION OF THE DRAWINGS AND SEQUENCE LISTINGS

FIG. 1 illustrates the ω-3/ω-6 fatty acid biosynthetic pathway.

FIG. 2 shows a comparison of the DNA sequence of the Phytophthora sojae Δ17 desaturase gene (designated as “PsD17”; SEQ ID NO:1) and the synthetic gene (designated as “PsD17S”; SEQ ID NO:3) codon-optimized for expression in Y. lipolytica.

FIG. 3 provides plasmid maps for the following: (A) pPsD17S; (B) pKUNF1-KEA; (C) pDMW214; and (D) pFmPs17S.

FIG. 4A diagrams the development of Yarrowia lipolytica strain Y2047, producing 11% ARA in the total lipid fraction. FIG. 4B provides a plasmid map for pKUNF12T6E, while FIG. 4C provides a plasmid map for pDMW271.

FIG. 5 provides a plasmid map for pZP3-P7U.

FIG. 6A diagrams the development of Yarrowia lipolytica strain Y4070, producing 12% ARA in the total lipid fraction. FIG. 6B provides a plasmid map for pZKLeuN-29E3, while FIG. 6C provides a plasmid map for pY116.

FIG. 7 provides plasmid maps for the following: (A) pKO2UF8289; and, (B) pZKSL-555R.

FIG. 8 shows a comparison of the DNA sequence of the Phytophthora ramorum Δ17 desaturase gene (designated as “PrD17”; SEQ ID NO:5) and the synthetic gene (designated as “PrD17S”; SEQ ID NO:7) codon-optimized for expression in Y. lipolytica.

FIG. 9 provides plasmid maps for the following: (A) pPrD17S; (B) pZUFPrD17S; and, (C) pZUF17.

The invention can be more fully understood from the following detailed description and the accompanying sequence descriptions, which form a part of this application.

The following sequences comply with 37 C.F.R. §1.821-1.825 (“Requirements for Patent Applications Containing Nucleotide Sequences and/or Amino Acid Sequence Disclosures—the Sequence Rules”) and are consistent with World Intellectual Property Organization (WIPO) Standard ST.25 (1998) and the sequence listing requirements of the EPO and PCT (Rules 5.2 and 49.5(a-bis), and Section 208 and Annex C of the Administrative Instructions). The symbols and format used for nucleotide and amino acid sequence data comply with the rules set forth in 37 C.F.R. §1.822.

A Sequence Listing is provided herewith on Compact Disk. The contents of the Compact Disk containing the Sequence Listing are hereby incorporated by reference in compliance with 37 C.F.R. §1.52(e). The Compact Disks are submitted in triplicate and are identical to one another. The disks are labeled “Copy 1—Sequence Listing”; “Copy 2—Sequence Listing”; and “CRF—Sequence Listing”. The disks contain the following file: CL3479 conversion listing_(—)25 having the following size: 230,000 bytes and which was created Apr. 17, 2007.

SEQ ID NOs:1-40 are ORFs encoding genes, proteins or plasmids, as identified in Table 1.

TABLE 1 Summary Of Gene And Protein SEQ ID Numbers Nucleic acid Protein Description and Abbreviation SEQ ID NO. SEQ ID NO. Phytophthora sojae Δ17 desaturase  1  2 (“PsD17”) (1092 bp) (363 AA) Synthetic Δ17 desaturase derived from  3  4 Phytophthora sojae, codon-optimized for (1086 bp) (361 AA) expression in Yarrowia lipolytica (“PsD17S”) Phytophthora ramorum Δ17 desaturase  5  6 (“PrD17”) (1086 bp) (361 AA) Synthetic Δ17 desaturase derived from  7  6 Phytophthora ramorum, codon-optimized (1086 bp) (361 AA) for expression in Yarrowia lipolytica (“PrD17S”) Phytophthora infestans Δ17 desaturase  8  9 (“PiD17”) (1086) bp) (361 AA) Plasmid pPsD17S 10 — (3806 bp) Plasmid pKunF1-KEA 11 — (6619 bp) Plasmid pDMW214 12 — (9513 bp) Plasmid pFmPsD17S 13 — (8727 bp) Plasmid pKUNF12T6E 14 — (12,649 bp)   Synthetic C_(18/20) elongase derived from 15 16 Thraustochytrium aureum (U.S. Pat. No.  (819 bp) (272 AA) 6,677,145), codon-optimized for expression in Yarrowia lipolytica (“EL2S”) Plasmid pDMW271 17 — (13,034 bp)   Synthetic Δ5 desaturase derived from 18 19 Homo sapiens (GenBank Accession No. (1335 bp) (444 AA) NP_037534), codon-optimized for expression in Yarrowia lipolytica Plasmid pZP3-P7U 20 — (8867 bp) Escherichia coli LoxP recombination site, 21 — recognized by a Cre recombinase enzyme  (34 bp) Plasmid pZKLeuN-29E3 22 — (14,655 bp)   Euglena gracilis Δ9 elongase (U.S. patent 31 24 applications No. 11/601563 and  (777 bp) (258 AA) 11/601564) (“EgD9e”) Synthetic Δ9 elongase derived from 23 24 Euglena gracilis (U.S. patent applications  (777 bp) (258 AA) No. 11/601563 and 11/601564), codon- optimized for expression in Yarrowia lipolytica (“EgD9eS”) Synthetic C_(16/18) elongase derived from 25 26 Mortierella alpina ELO3 (U.S. patent  (828 bp) (275 AA) application No. 11/253882), codon- optimized for expression in Yarrowia lipolytica (“ME3S”) Plasmid pY116 27 — (8739 bp) Plasmid pKO2UF8289 28 — (15,304 bp)   Synthetic mutant Δ8 desaturase, derived 29 30 from Euglena gracilis (U.S. patent (1272 bp) (422 AA) application No. 11/635258) (“Mutant EgD8S-23”) Plasmid pZKSL-555R 32 — (13,707 bp)   Euglena gracilis Δ5 desaturase (U.S. 37 34 patent application No. 60/801172) (1350 bp) (449 AA) (“EgD5”) Synthetic Δ5 desaturase derived from 33 34 Euglena gracilis (U.S. patent application (1350 bp) (449 AA) No. 60/801172), codon-optimized for expression in Yarrowia lipolytica (“EgD5S”) Synthetic Δ5 desaturase derived from 35 36 Peridinium sp. CCMP626 (U.S. patent (1392 bp) (463 AA) application No. 60/801119), codon- optimized for expression in Yarrowia lipolytica (“RD5S”) Plasmid pPrD17S 38 — (3806 bp) Plasmid pZUF17 39 — (8165 bp) Plasmid pZUFPrD17S 40 — (8174 bp)

DETAILED DESCRIPTION OF THE INVENTION

All patents, patent applications, and publications cited herein are incorporated by reference in their entirety. This specifically includes the following Applicants' Assignee's co-pending applications: U.S. Pat. No. 7,125,672, U.S. Pat. No. 7,189,559, U.S. Pat. No. 7,192,762, U.S. patent application Ser. No. 10/840,579 and Ser. No. 10/840,325 (filed May 6, 2004), U.S. patent application Ser. No. 10/869,630 (filed Jun. 16, 2004), U.S. patent application Ser. No. 10/882,760 (filed Jul. 1, 2004), U.S. patent application Ser. No. 10/985,254 and Ser. No. 10/985,691 (filed Nov. 10, 2004), U.S. patent application Ser. No. 10/987,548 (filed Nov. 12, 2004), U.S. patent application Ser. No. 11/024,545 and Ser. No. 11/024,544 (filed Dec. 29, 2004), U.S. patent application Ser. No. 11/166,993 (filed Jun. 24, 2005), U.S. patent application Ser. No. 11/183,664 (filed Jul. 18, 2005), U.S. patent application Ser. No. 11/185,301 (filed Jul. 20, 2005), U.S. patent application Ser. No. 11/190,750 (filed Jul. 27, 2005), U.S. patent application Ser. No. 11/198,975 (filed Aug. 8, 2005), U.S. patent application Ser. No. 11/225,354 (filed Sep. 13, 2005), U.S. patent application Ser. No. 11/253,882 (filed Oct. 19, 2005), U.S. patent application Ser. No. 11/264,784 and Ser. No. 11/264,737 (filed Nov. 1, 2005), U.S. patent application Ser. No. 11/265,761 (filed Nov. 2, 2005), U.S. Patent Application No. 60/795,810 (filed Apr. 28, 2006), U.S. Patent Application No. 60/793,575 (filed Apr. 20, 2006), U.S. Patent Application No. 60/796,637 (filed May 2, 2006), U.S. Patent Applications No. 60/801,172 and No. 60/801,119 (filed May 17, 2006), U.S. Patent Application No. 60/853,563 (filed Oct. 23, 2006), U.S. Patent Application No. 60/855,177 (filed Oct. 30, 2006), U.S. patent application Ser. No. 11/601,563 and Ser. No. 11/601,564 (filed Nov. 16, 2006), U.S. patent application Ser. No. 11/635,258 (filed Dec. 7, 2006) and U.S. patent application Ser. No. 11/613,420 (filed Dec. 20, 2006).

In accordance with the subject invention, Applicants identify novel Oomycota Δ17 desaturase enzymes and genes encoding the same that may be used for the manipulation of biochemical pathways for the production of healthful PUFAs. Thus, the subject invention finds many applications. PUFAs, or derivatives thereof, made by the methodology disclosed herein can be used as dietary substitutes, or supplements, particularly infant formulas, for patients undergoing intravenous feeding or for preventing or treating malnutrition. Alternatively, the purified PUFAs (or derivatives thereof) may be incorporated into cooking oils, fats or margarines formulated so that in normal use the recipient would receive the desired amount for dietary supplementation. The PUFAs may also be incorporated into infant formulas, nutritional supplements or other food products and may find use as anti-inflammatory or cholesterol lowering agents. Optionally, the compositions may be used for pharmaceutical use (human or veterinary).

Supplementation of humans or animals with PUFAs produced by recombinant means can result in increased levels of the added PUFAs, as well as their metabolic progeny. For example, treatment with EPA can result not only in increased levels of EPA, but also downstream products of EPA such as eicosanoids (i.e., prostaglandins, leukotrienes, thromboxanes). Complex regulatory mechanisms can make it desirable to combine various PUFAs, or add different conjugates of PUFAs, in order to prevent, control or overcome such mechanisms to achieve the desired levels of specific PUFAs in an individual.

Definitions

In this disclosure, a number of terms and abbreviations are used. The following definitions are provided.

“Open reading frame” is abbreviated ORF.

“Polymerase chain reaction” is abbreviated PCR.

“American Type Culture Collection” is abbreviated ATCC.

“Polyunsaturated fatty acid(s)” is abbreviated PUFA(s).

“Triacylglycerols” are abbreviated TAGs.

As used herein the term “invention” or “present invention” is intended to refer to all aspects and embodiments of the invention as described in the claims and specification herein and should not be read so as to be limited to any particular embodiment or aspect.

The term “fatty acids” refers to long chain aliphatic acids (alkanoic acids) of varying chain lengths, from about C₁₂ to C₂₂ (although both longer and shorter chain-length acids are known). The predominant chain lengths are between C₁₆ and C₂₂. The structure of a fatty acid is represented by a simple notation system of “X:Y”, where X is the total number of carbon (C) atoms in the particular fatty acid and Y is the number of double bonds. Additional details concerning the differentiation between “saturated fatty acids” versus “unsaturated fatty acids”, “monounsaturated fatty acids” versus “polyunsaturated fatty acids” (or “PUFAs”), and “omega-6 fatty acids” (ω-6 or n-6) versus “omega-3 fatty acids” (ω-3 or n-3) are provided in PCT Publication No. WO 2004/101757.

Nomenclature used to describe PUFAs in the present disclosure is shown below in Table 2. In the column titled “Shorthand Notation”, the omega-reference system is used to indicate the number of carbons, the number of double bonds and the position of the double bond closest to the omega carbon, counting from the omega carbon (which is numbered 1 for this purpose). The remainder of the Table summarizes the common names of ω-3 and ω-6 fatty acids and their precursors, the abbreviations that will be used throughout the specification and each compounds' chemical name.

TABLE 2 Nomenclature of Polyunsaturated Fatty Acids And Precursors Shorthand Common Name Abbreviation Chemical Name Notation Myristic — tetradecanoic 14:0 Palmitic Palmitate hexadecanoic 16:0 Palmitoleic — 9-hexadecenoic 16:1 Stearic — octadecanoic 18:0 Oleic — cis-9-octadecenoic 18:1 Linoleic LA cis-9,12-octadecadienoic 18:2 ω-6 γ-Linoleic GLA cis-6,9,12- 18:3 ω-6 octadecatrienoic Eicosadienoic EDA cis-11,14-eicosadienoic 20:2 ω-6 Dihomo-γ- DGLA cis-8,11,14- 20:3 ω-6 Linoleic eicosatrienoic Arachidonic ARA cis-5,8,11,14- 20:4 ω-6 eicosatetraenoic α-Linolenic ALA cis-9,12,15- 18:3 ω-3 octadecatrienoic Stearidonic STA cis-6,9,12,15- 18:4 ω-3 octadecatetraenoic Eicosatrienoic ETrA cis-11,14,17- 20:3 ω-3 eicosatrienoic Eicosatetraenoic ETA cis-8,11,14,17- 20:4 ω-3 eicosatetraenoic Eicosapentaenoic EPA cis-5,8,11,14,17- 20:5 ω-3 eicosapentaenoic Docosapentaenoic DPA cis-7,10,13,16,19- 22:5 ω-3 docosapentaenoic Docosahexaenoic DHA cis-4,7,10,13,16,19- 22:6 ω-3 docosahexaenoic

The terms “triacylglycerol”, “oil” and “TAGs” refer to neutral lipids composed of three fatty acyl residues esterified to a glycerol molecule (and such terms will be used interchangeably throughout the present disclosure herein). Such oils can contain long chain PUFAs, as well as shorter saturated and unsaturated fatty acids and longer chain saturated fatty acids. Thus, “oil biosynthesis” generically refers to the synthesis of TAGs in the cell. “Microbial oils” or “single cell oils” are those oils naturally produced by microorganisms during their lifespan.

“Percent (%) PUFAs in the total lipid and oil fractions” refers to the percent of PUFAs relative to the total fatty acids in those fractions. The term “total lipid fraction” or “lipid fraction” both refer to the sum of all lipids (i.e., neutral and polar) within an oleaginous organism, thus including those lipids that are located in the phosphatidylcholine (PC) fraction, phosphatidyletanolamine (PE) fraction and triacylglycerol (TAG or oil) fraction. However, the terms “lipid” and “oil” will be used interchangeably throughout the specification.

A “metabolic pathway”, or “biosynthetic pathway”, in a biochemical sense, can be regarded as a series of chemical reactions occurring within a cell, catalyzed by enzymes, to achieve either the formation of a metabolic product to be used or stored by the cell, or the initiation of another metabolic pathway (then called a flux generating step). Many of these pathways are elaborate, and involve a step by step modification of the initial substance to shape it into a product having the exact chemical structure desired.

The term “PUFA biosynthetic pathway” refers to a metabolic process that converts oleic acid to LA, EDA, GLA, DGLA, ARA, ALA, STA, ETrA, ETA, EPA, DPA and DHA. This process is well described in the literature (e.g., see PCT Publication No. WO2006/052870). Briefly, this process involves elongation of the carbon chain through the addition of carbon atoms and desaturation of the molecule through the addition of double bonds, via a series of special desaturation and elongation enzymes (i.e., “PUFA biosynthetic pathway enzymes”) present in the endoplasmic reticulim membrane. More specifically, “PUFA biosynthetic pathway enzymes” refer to any of the following enzymes (and genes which encode said enzymes) associated with the biosynthesis of a PUFA, including: a Δ4 desaturase, a Δ5 desaturase, a Δ6 desaturase, a Δ12 desaturase, a Δ15 desaturase, a Δ17 desaturase, a Δ9 desaturase, a Δ8 desaturase, a Δ9 elongase, a C_(14/16) elongase, a C_(16/18) elongase, a C_(18/20) elongase and/or a C_(20/22) elongase.

The term “ω-3/ω-6 fatty acid biosynthetic pathway” refers to a set of genes which, when expressed under the appropriate conditions encode enzymes that catalyze the production of either or both ω-3 and ω-6 fatty acids. Typically the genes involved in the ω-3/ω-6 fatty acid biosynthetic pathway encode PUFA biosynthetic pathway enzymes. A representative pathway is illustrated in FIG. 1, providing for the conversion of oleic acid through various intermediates to DHA, which demonstrates how both ω-3 and ω-6 fatty acids may be produced from a common source. The pathway is naturally divided into two portions where one portion will generate ω-3 fatty acids and the other portion, only ω-6 fatty acids. That portion that only generates ω-3 fatty acids will be referred to herein as the ω-3 fatty acid biosynthetic pathway, whereas that portion that generates only ω-6 fatty acids will be referred to herein as the ω-6 fatty acid biosynthetic pathway.

The term “functional” as used herein in context with the ω-3/ω-6 fatty acid biosynthetic pathway means that some (or all of) the genes in the pathway express active enzymes, resulting in in vivo catalysis or substrate conversion. It should be understood that “ω-3/ω-6 fatty acid biosynthetic pathway” or “functional ω-3/ω-6 fatty acid biosynthetic pathway” does not imply that all the genes listed in the above paragraph are required, as a number of fatty acid products will only require the expression of a subset of the genes of this pathway.

The term “desaturase” refers to a polypeptide that can desaturate, i.e., introduce a double bond, in one or more fatty acids to produce a fatty acid or precursor of interest. Despite use of the omega-reference system throughout the specification to refer to specific fatty acids, it is more convenient to indicate the activity of a desaturase by counting from the carboxyl end of the substrate using the delta-system. Of interest herein are: 1.) Δ8 desaturases that will catalyze the conversion of EDA to DGLA and/or ETrA to ETA; 2.) Δ5 desaturases that catalyze the conversion of DGLA to ARA and/or ETA to EPA; 3.) Δ6 desaturases that catalyze the conversion of LA to GLA and/or ALA to STA; 4.) Δ4 desaturases that catalyze the conversion of DPA to DHA; 5.) Δ12 desaturases that catalyze the conversion of oleic acid to LA; 6.) Δ15 desaturases that catalyze the conversion of LA to ALA and/or GLA to STA; and 7.) Δ9 desaturases that catalyze the conversion of palmitate to palmitoleic acid (16:1) and/or stearate to oleic acid (18:1).

Of particular interest herein are Δ17 desaturases that desaturate a fatty acid between the 17^(th) and 18^(th) carbon atom numbered from the carboxyl-terminal end of the molecule and which, for example, catalyze the conversion of ARA to EPA (and optionally DGLA to ETA). In the art, Δ17 desaturases (and also Δ15 desaturases) are also occasionally referred to as “omega-3 desaturases”, “w-3 desaturases”, and/or “ω-3 desaturases”, based on their ability to convert ω-6 fatty acids into their ω-3 counterparts (e.g., conversion of LA into ALA or DGLA into ETA and ARA into EPA, respectively).

Some desaturases have activity on two or more substrates. Based on this ability, these enzymes can be further classified with respect to their desaturase activities as being either “monofunctional” or “bifunctional”. In some embodiments, it is most desirable to empirically determine the specificity of a fatty acid desaturase by transforming a suitable host with the gene for the fatty acid desaturase and determining its effect on the fatty acid profile of the host.

More specifically, Δ17 desaturases are defined herein as those fatty acid desaturases having monofunctional or bifunctional Δ17 desaturase activity, wherein Δ17 desaturase activity is the conversion of ARA to EPA and/or DGLA to ETA. The term “monofunctional Δ17 desaturase”, “monofunctional Δ17 desaturase activity” or “exclusive Δ17 desaturase activity” refers to a Δ17 desaturase that is capable of converting ARA to EPA and/or DGLA to ETA but not LA to ALA. In contrast, “bifunctional Δ17 desaturase”, “bifunctional Δ17 desaturase activity” or “primary Δ17 desaturase activity” refers to a Δ17 desaturase that preferentially converts ARA to EPA and/or DGLA to ETA but additionally has limited ability to convert LA into ALA (thus exhibiting primarily Δ17 desaturase activity and limited Δ15 desaturase activity).

It should be noted that Δ17 desaturases can have specificities other than Δ17 and Δ15 desaturation that are not relevant in this classification. It should also be noted that the distinction between monofunctional and bifunctional Δ17 desaturases is a practical one and not absolute; e.g., the same enzyme may function with either monofunctional or bifunctional Δ17 desaturase activity, depending on the level of its expression or growth condition.

For the purposes herein, the term “PsD17” refers to a Δ17 desaturase enzyme (SEQ ID NO:2) isolated from Phytophthora sojae, encoded by SEQ ID NO:1. In contrast, the term “PsD17S” refers to a synthetic Δ17 desaturase derived from Phytophthora sojae that is codon-optimized for expression in Yarrowia lipolytica (i.e., SEQ ID NOs:3 and 4). Based on analyses described herein, PsD17 and PsD17S are further classified as bifunctional Δ17 desaturases.

Similarly, the term “PrD17” refers to a Δ17 desaturase enzyme (SEQ ID NO:6) isolated from Phytophthora ramorum, encoded by SEQ ID NO:5. In contrast, the term “PrD17S” refers to a synthetic Δ17 desaturase derived from Phytophthora ramorum that is codon-optimized for expression in Yarrowia lipolytica (i.e., SEQ ID NOs:7 and 6). Based on analyses described herein, PrD17 and PrD17S are further classified as monofunctional Δ17 desaturases.

The terms “conversion efficiency” and “percent substrate conversion” refer to the efficiency by which a particular enzyme (e.g., a desaturase) can convert substrate to product. The conversion efficiency is measured according to the following formula: ([product]/[substrate+product])*100, where ‘product’ includes the immediate product and all products in the pathway derived from it.

The term “elongase” refers to a polypeptide that can elongate a fatty acid carbon chain to produce an acid that is 2 carbons longer than the fatty acid substrate that the elongase acts upon. This process of elongation occurs in a multi-step mechanism in association with fatty acid synthase, as described in PCT Publication No. WO 2004/101757. Examples of reactions catalyzed by elongase systems are the conversion of GLA to DGLA, STA to ETA and EPA to DPA. In general, the substrate selectivity of elongases is somewhat broad but segregated by both chain length and the degree and type of unsaturation. For example, a C_(14/16) elongase will utilize a C₁₄ substrate (e.g., myristic acid), a C_(16/18) elongase will utilize a C₁₆ substrate (e.g., palmitate), a C_(18/20) elongase (also known as a Δ6 elongase as the terms can be used interchangeably) will utilize a C₁₈ substrate (e.g., GLA, STA) and a C_(20/22) elongase will utilize a C₂₀ substrate (e.g., EPA). In like manner, a Δ9 elongase is able to catalyze the conversion of LA and ALA to EDA and ETrA, respectively. It is important to note that some elongases have broad specificity and thus a single enzyme may be capable of catalyzing several elongase reactions (e.g., thereby acting as both a C_(16/18) elongase and a C_(18/20) elongase).

The term “oomycetes” refers to a group of heterotrophic organisms generally known as the water molds and downy mildews. They are filamentous protists that must absorb their food from the surrounding water or soil, or may invade the body of another organism to feed. As such, oomycetes play an important role in the decomposition and recycling of decaying matter. Phytophthora is a genus of the Oomycetes, comprising fifty-nine different species. Although oomycetes have similarities to fungi through convergent evolution, they are not fungi (as previously thought); instead, the oomycetes are part of the kingdom Stramenopiles and are thereby distinct from plants, fungi and animals. Diatoms and golden-brown and brown algae (e.g., kelp) are also included within kingdom Stramenopiles.

The term “oleaginous” refers to those organisms that tend to store their energy source in the form of lipid (Weete, In: Fungal Lipid Biochemistry, 2^(nd) Ed., Plenum, 1980). The term “oleaginous yeast” refers to those microorganisms classified as yeasts that can make oil. Generally, the cellular oil or TAG content of oleaginous microorganisms follows a sigmoid curve, wherein the concentration of lipid increases until it reaches a maximum at the late logarithmic or early stationary growth phase and then gradually decreases during the late stationary and death phases (Yongmanitchai and Ward, Appl. Environ. Microbiol., 57:419-25 (1991)). It is not uncommon for oleaginous microorganisms to accumulate in excess of about 25% of their dry cell weight as oil. Examples of oleaginous yeast include, but are no means limited to, the following genera: Yarrowia, Candida, Rhodotorula, Rhodosporidium, Cryptococcus, Trichosporon and Lipomyces.

As used herein, an “isolated nucleic acid fragment” or “isolated nucleic acid molecule” will be used interchangeably and refers to a polymer of RNA or DNA that is single- or double-stranded, optionally containing synthetic, non-natural or altered nucleotide bases. An isolated nucleic acid fragment in the form of a polymer of DNA may be comprised of one or more segments of cDNA, genomic DNA or synthetic DNA.

A nucleic acid fragment is “hybridizable” to another nucleic acid fragment, such as a cDNA, genomic DNA, or RNA molecule, when a single-stranded form of the nucleic acid fragment can anneal to the other nucleic acid fragment under the appropriate conditions of temperature and solution ionic strength. Hybridization and washing conditions are well known and exemplified in Sambrook, J., Fritsch, E. F. and Maniatis, T. Molecular Cloning: A Laboratory Manual, 2^(nd) ed., Cold Spring Harbor Laboratory: Cold Spring Harbor, N.Y. (1989), particularly Chapter 11 and Table 11.1 therein (entirely incorporated herein by reference). The conditions of temperature and ionic strength determine the “stringency” of the hybridization. Stringency conditions can be adjusted to screen for moderately similar fragments (such as homologous sequences from distantly related organisms), to highly similar fragments (such as genes that duplicate functional enzymes from closely related organisms). Post-hybridization washes determine stringency conditions. One set of preferred conditions uses a series of washes starting with 6×SSC, 0.5% SDS at room temperature for 15 min, then repeated with 2×SSC, 0.5% SDS at 45° C. for 30 min, and then repeated twice with 0.2×SSC, 0.5% SDS at 50° C. for 30 min. A more preferred set of stringent conditions uses higher temperatures in which the washes are identical to those above except for the temperature of the final two 30 min washes in 0.2×SSC, 0.5% SDS was increased to 60° C. Another preferred set of highly stringent conditions uses two final washes in 0.1×SSC, 0.1% SDS at 65° C. An additional set of stringent conditions include hybridization at 0.1×SSC, 0.1% SDS, 65° C. and washes with 2×SSC, 0.1% SDS followed by 0.1×SSC, 0.1% SDS, for example.

Hybridization requires that the two nucleic acids contain complementary sequences, although depending on the stringency of the hybridization, mismatches between bases are possible. The appropriate stringency for hybridizing nucleic acids depends on the length of the nucleic acids and the degree of complementation, variables well known in the art. The greater the degree of similarity or homology between two nucleotide sequences, the greater the value of Tm for hybrids of nucleic acids having those sequences. The relative stability (corresponding to higher Tm) of nucleic acid hybridizations decreases in the following order: RNA:RNA, DNA:RNA, DNA:DNA. For hybrids of greater than 100 nucleotides in length, equations for calculating Tm have been derived (see Sambrook et al., supra, 9.50-9.51). For hybridizations with shorter nucleic acids, i.e., oligonucleotides, the position of mismatches becomes more important, and the length of the oligonucleotide determines its specificity (see Sambrook et al., supra, 11.7-11.8). In one embodiment the length for a hybridizable nucleic acid is at least about 10 nucleotides. Preferably a minimum length for a hybridizable nucleic acid is at least about 15 nucleotides; more preferably at least about 20 nucleotides; and most preferably the length is at least about 30 nucleotides. Furthermore, the skilled artisan will recognize that the temperature and wash solution salt concentration may be adjusted as necessary according to factors such as length of the probe.

A “substantial portion” of an amino acid or nucleotide sequence is that portion comprising enough of the amino acid sequence of a polypeptide or the nucleotide sequence of a gene to putatively identify that polypeptide or gene, either by manual evaluation of the sequence by one skilled in the art, or by computer-automated sequence comparison and identification using algorithms such as BLAST (Basic Local Alignment Search Tool; Altschul, S. F., et al., J. Mol. Biol. 215:403-410 (1993)). In general, a sequence of ten or more contiguous amino acids or thirty or more nucleotides is necessary in order to putatively identify a polypeptide or nucleic acid sequence as homologous to a known protein or gene. Moreover, with respect to nucleotide sequences, gene specific oligonucleotide probes comprising 20-30 contiguous nucleotides may be used in sequence-dependent methods of gene identification (e.g., Southern hybridization) and isolation (e.g., in situ hybridization of bacterial colonies or bacteriophage plaques). In addition, short oligonucleotides of 12-15 bases may be used as amplification primers in PCR in order to obtain a particular nucleic acid fragment comprising the primers. Accordingly, a “substantial portion” of a nucleotide sequence comprises enough of the sequence to specifically identify and/or isolate a nucleic acid fragment comprising the sequence. The instant specification teaches the complete amino acid and nucleotide sequence encoding particular Oomycete proteins. The skilled artisan, having the benefit of the sequences as reported herein, may now use all or a substantial portion of the disclosed sequences for purposes known to those skilled in this art. Accordingly, the instant invention comprises the complete sequences as reported in the accompanying Sequence Listing, as well as substantial portions of those sequences as defined above.

The term “complementary” is used to describe the relationship between nucleotide bases that are capable of hybridizing to one another. For example, with respect to DNA, adenosine is complementary to thymine and cytosine is complementary to guanine. Accordingly, the instant invention also includes isolated nucleic acid fragments that are complementary to the complete sequences as reported in the accompanying Sequence Listing, as well as those substantially similar nucleic acid sequences.

The terms “homology” and “homologous” are used interchangeably herein. They refer to nucleic acid fragments wherein changes in one or more nucleotide bases do not affect the ability of the nucleic acid fragment to mediate gene expression or produce a certain phenotype. These terms also refer to modifications of the nucleic acid fragments of the instant invention such as deletion or insertion of one or more nucleotides that do not substantially alter the functional properties of the resulting nucleic acid fragment relative to the initial, unmodified fragment. It is therefore understood, as those skilled in the art will appreciate, that the invention encompasses more than the specific exemplary sequences.

Moreover, the skilled artisan recognizes that homologous nucleic acid sequences encompassed by this invention are also defined by their ability to hybridize, under moderately stringent conditions (e.g., 0.5×SSC, 0.1% SDS, 60° C.) with the sequences exemplified herein, or to any portion of the nucleotide sequences disclosed herein and which are functionally equivalent to any of the nucleic acid sequences disclosed herein.

“Codon degeneracy” refers to the nature in the genetic code permitting variation of the nucleotide sequence without effecting the amino acid sequence of an encoded polypeptide. The skilled artisan is well aware of the “codon-bias” exhibited by a specific host cell in usage of nucleotide codons to specify a given amino acid. Therefore, when synthesizing a gene for improved expression in a host cell, it is desirable to design the gene such that its frequency of codon usage approaches the frequency of preferred codon usage of the host cell.

“Chemically synthesized”, as related to a sequence of DNA, means that the component nucleotides were assembled in vitro. Manual chemical synthesis of DNA may be accomplished using well-established procedures or, automated chemical synthesis can be performed using one of a number of commercially available machines. “Synthetic genes” can be assembled from oligonucleotide building blocks that are chemically synthesized using procedures known to those skilled in the art. These building blocks are ligated and annealed to form gene segments that are then enzymatically assembled to construct the entire gene. Accordingly, the genes can be tailored for optimal gene expression based on optimization of nucleotide sequence to reflect the codon bias of the host cell. The skilled artisan appreciates the likelihood of successful gene expression if codon usage is biased towards those codons favored by the host. Determination of preferred codons can be based on a survey of genes derived from the host cell, where sequence information is available.

“Gene” refers to a nucleic acid fragment that expresses a specific protein, and that may refer to the coding region alone or may include regulatory sequences preceding (5′ non-coding sequences) and following (3′ non-coding sequences) the coding sequence. “Native gene” refers to a gene as found in nature with its own regulatory sequences. “Chimeric gene” refers to any gene that is not a native gene, comprising regulatory and coding sequences that are not found together in nature. Accordingly, a chimeric gene may comprise regulatory sequences and coding sequences that are derived from different sources, or regulatory sequences and coding sequences derived from the same source, but arranged in a manner different than that found in nature. “Endogenous gene” refers to a native gene in its natural location in the genome of an organism. A “foreign” gene refers to a gene that is introduced into the host organism by gene transfer. Foreign genes can comprise native genes inserted into a non-native organism, native genes introduced into a new location within the native host, or chimeric genes. A “transgene” is a gene that has been introduced into the genome by a transformation procedure. A “codon-optimized gene” is a gene having its frequency of codon usage designed to mimic the frequency of preferred codon usage of the host cell.

“Coding sequence” refers to a DNA sequence that codes for a specific amino acid sequence. “Suitable regulatory sequences” refer to nucleotide sequences located upstream (5′ non-coding sequences), within, or downstream (3′ non-coding sequences) of a coding sequence, and which influence the transcription, RNA processing or stability, or translation of the associated coding sequence. Regulatory sequences may include promoters, translation leader sequences, introns, polyadenylation recognition sequences, RNA processing sites, effector binding sites and stem-loop structures.

“Promoter” refers to a DNA sequence capable of controlling the expression of a coding sequence or functional RNA. In general, a coding sequence is located 3′ to a promoter sequence. Promoters may be derived in their entirety from a native gene, or be composed of different elements derived from different promoters found in nature, or even comprise synthetic DNA segments. It is understood by those skilled in the art that different promoters may direct the expression of a gene in different tissues or cell types, or at different stages of development, or in response to different environmental or physiological conditions. Promoters that cause a gene to be expressed in most cell types at most times are commonly referred to as “constitutive promoters”. It is further recognized that since in most cases the exact boundaries of regulatory sequences have not been completely defined, DNA fragments of different lengths may have identical promoter activity.

The terms “3′ non-coding sequences” and “transcription terminator” refer to DNA sequences located downstream of a coding sequence. This includes polyadenylation recognition sequences and other sequences encoding regulatory signals capable of affecting mRNA processing or gene expression. The polyadenylation signal is usually characterized by affecting the addition of polyadenylic acid tracts to the 3′ end of the mRNA precursor. The 3′ region can influence the transcription, RNA processing or stability, or translation of the associated coding sequence.

“RNA transcript” refers to the product resulting from RNA polymerase-catalyzed transcription of a DNA sequence. When the RNA transcript is a perfect complementary copy of the DNA sequence, it is referred to as the primary transcript or it may be a RNA sequence derived from post-transcriptional processing of the primary transcript and is referred to as the mature RNA. “Messenger RNA” or “mRNA” refers to the RNA that is without introns and that can be translated into protein by the cell. “cDNA” refers to a double-stranded DNA that is complementary to, and derived from, mRNA. “Sense” RNA refers to RNA transcript that includes the mRNA and so can be translated into protein by the cell. “Antisense RNA” refers to a RNA transcript that is complementary to all or part of a target primary transcript or mRNA and that blocks the expression of a target gene (U.S. Pat. No. 5,107,065; PCT Publication No. WO 99/28508). The complementarity of an antisense RNA may be with any part of the specific gene transcript, i.e., at the 5′ non-coding sequence, 3′ non-coding sequence, or the coding sequence. “Functional RNA” refers to antisense RNA, ribozyme RNA, or other RNA that is not translated and yet has an effect on cellular processes.

The term “operably linked” refers to the association of nucleic acid sequences on a single nucleic acid fragment so that the function of one is affected by the other. For example, a promoter is operably linked with a coding sequence when it is capable of affecting the expression of that coding sequence (i.e., the coding sequence is under the transcriptional control of the promoter). Coding sequences can be operably linked to regulatory sequences in sense or antisense orientation.

The term “expression”, as used herein, refers to the transcription and stable accumulation of sense (mRNA) or antisense RNA derived from the nucleic acid fragments of the invention. Expression may also refer to translation of mRNA into a polypeptide.

“Mature” protein refers to a post-translationally processed polypeptide, i.e., one from which any pre- or propeptides present in the primary translation product have been removed. “Precursor” protein refers to the primary product of translation of mRNA, i.e., with pre- and propeptides still present. Pre- and propeptides may be (but are not limited to) intracellular localization signals.

“Transformation” refers to the transfer of a nucleic acid molecule into a host organism, resulting in genetically stable inheritance. The nucleic acid molecule may be a plasmid that replicates autonomously, for example, or, it may integrate into the genome of the host organism. Host organisms containing the transformed nucleic acid fragments are referred to as “transgenic” or “recombinant” or “transformed” organisms.

The terms “plasmid”, “vector” and “cassette” refer to an extra chromosomal element often carrying genes that are not part of the central metabolism of the cell, and usually in the form of circular double-stranded DNA fragments. Such elements may be autonomously replicating sequences, genome integrating sequences, phage or nucleotide sequences, linear or circular, of a single- or double-stranded DNA or RNA, derived from any source, in which a number of nucleotide sequences have been joined or recombined into a unique construction which is capable of introducing a promoter fragment and DNA sequence for a selected gene product along with appropriate 3′ untranslated sequence into a cell. “Expression cassette” refers to a specific vector containing a foreign gene and having elements in addition to the foreign gene that allow for enhanced expression of that gene in a foreign host.

The term “percent identity”, as known in the art, is a relationship between two or more polypeptide sequences or two or more polynucleotide sequences, as determined by comparing the sequences. In the art, “identity” also means the degree of sequence relatedness between polypeptide or polynucleotide sequences, as the case may be, as determined by the match between strings of such sequences. “Identity” and “similarity” can be readily calculated by known methods, including but not limited to those described in: 1.) Computational Molecular Biology (Lesk, A. M., Ed.) Oxford University: NY (1988); 2.) Biocomputing: Informatics and Genome Projects (Smith, D. W., Ed.) Academic: NY (1993); 3.) Computer Analysis of Sequence Data, Part I (Griffin, A. M., and Griffin, H. G., Eds.) Humania: NJ (1994); 4.) Sequence Analysis in Molecular Biology (von Heinje, G., Ed.) Academic (1987); and 5.) Sequence Analysis Primer (Gribskov, M. and Devereux, J., Eds.) Stockton: NY (1991).

Preferred methods to determine identity are designed to give the best match between the sequences tested. Methods to determine identity and similarity are codified in publicly available computer programs. Sequence alignments and percent identity calculations may be performed using the MegAlign™ program of the LASERGENE bioinformatics computing suite (DNASTAR Inc., Madison, Wis.). Multiple alignment of the sequences is performed using the “Clustal method of alignment” which encompasses several varieties of the algorithm including the “Clustal V method of alignment” corresponding to the alignment method labeled Clustal V (described by Higgins and Sharp, CABIOS. 5:151-153 (1989); Higgins, D. G. et al. (1992) Comput. Appl. Biosci. 8:189-191) and found in the MegAlign program of the LASERGENE bioinformatics computing suite (DNASTAR Inc., Madison, Wis.). For multiple alignments, the default values correspond to GAP PENALTY=10 and GAP LENGTH PENALTY=10. Default parameters for pairwise alignments and calculation of percent identity of protein sequences using the Clustal method are KTUPLE=1, GAP PENALTY=3, WINDOW=5 and DIAGONALS SAVED=5. For nucleic acids these parameters are KTUPLE=2, GAP PENALTY=5, WINDOW=4 and DIAGONALS SAVED=4. After alignment of the sequences using the Clustal V program, it is possible to obtain a “percent identity” by viewing the “sequence distances” table in the same program. Additionally the “Clustal W method of alignment” is available and corresponds to the alignment method labeled Clustal W (described by Higgins and Sharp, CABIOS. 5:151-153 (1989); Higgins, D. G. et al. (1992) Comput. Appl. Biosci. 8:189-191) and found in the MegAlign v6.1 program of the LASERGENE bioinformatics computing suite (DNASTAR Inc., Madison, Wis.). Default parameters for multiple alignment (GAP PENALTY=10, GAP LENGTH PENALTY=0.2, Delay Divergen Seqs(%)=30, DNA Transition Weight=0.5, Protein Weight Matrix=Gonnet Series, DNA Weight Matrix=IUB). After alignment of the sequences using the Clustal V program, it is possible to obtain a “percent identity” by viewing the “sequence distances” table in the same program.

“BLASTN method of alignment” is an algorithm provided by the National Center for Biotechnology Information (NCBI) to compare nucleotide sequences using default parameters.

It is well understood by one skilled in the art that many levels of sequence identity are useful in identifying polypeptides, from other species, wherein such polypeptides have the same or similar function or activity. Suitable nucleic acid fragments (isolated polynucleotides of the present invention) encode polypeptides that are at least about 70% identical, preferably at least about 75% identical, and more preferably at least about 80% identical to the amino acid sequences reported herein. Preferred nucleic acid fragments encode amino acid sequences that are at least about 85% identical to the amino acid sequences reported herein. More preferred nucleic acid fragments encode amino acid sequences that are at least about 90% identical to the amino acid sequences reported herein. Most preferred are nucleic acid fragments that encode amino acid sequences that are at least about 95% identical to the amino acid sequences reported herein. Indeed, any integer amino acid identity from 70% to 100% may be useful in describing the present invention, such as 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99%. Suitable nucleic acid fragments not only have the above homologies but typically encode a polypeptide having at least 50 amino acids, preferably at least 100 amino acids, more preferably at least 150 amino acids, still more preferably at least 200 amino acids, and most preferably at least 250 amino acids.

The term “sequence analysis software” refers to any computer algorithm or software program that is useful for the analysis of nucleotide or amino acid sequences. “Sequence analysis software” may be commercially available or independently developed. Typical sequence analysis software will include, but is not limited to: 1.) the GCG suite of programs (Wisconsin Package Version 9.0, Genetics Computer Group (GCG), Madison, Wis.); 2.) BLASTP, BLASTN, BLASTX (Altschul et al., J. Mol. Biol., 215:403-410 (1990)); 3.) DNASTAR (DNASTAR, Inc. Madison, Wis.); 4.) Sequencher (Gene Codes Corporation, Ann Arbor, Mich.); and 5.) the FASTA program incorporating the Smith-Waterman algorithm (W. R. Pearson, Comput. Methods Genome Res., [Proc. Int. Symp.] (1994), Meeting Date 1992, 111-20. Editor(s): Suhai, Sandor. Plenum: New York, N.Y.). Within the context of this application it will be understood that where sequence analysis software is used for analysis, that the results of the analysis will be based on the “default values” of the program referenced, unless otherwise specified. As used herein “default values” will mean any set of values or parameters that originally load with the software when first initialized.

Standard recombinant DNA and molecular cloning techniques used herein are well known in the art and are described by Sambrook, J., Fritsch, E. F. and Maniatis, T., Molecular Cloning: A Laboratory Manual, 2^(nd) ed., Cold Spring Harbor Laboratory: Cold Spring Harbor, N.Y. (1989) (hereinafter “Maniatis”); by Silhavy, T. J., Bennan, M. L. and Enquist, L. W., Experiments with Gene Fusions, Cold Spring Harbor Laboratory: Cold Spring Harbor, N.Y. (1984); and by Ausubel, F. M. et al., Current Protocols in Molecular Biology, published by Greene Publishing Assoc. and Wiley-Interscience, Hoboken, N.J. (1987).

An Overview: Microbial Biosynthesis of Fatty Acids and Triacylglycerols

In general, lipid accumulation in oleaginous microorganisms is triggered in response to the overall carbon to nitrogen ratio present in the growth medium. This process, leading to the de novo synthesis of free palmitate (16:0) in oleaginous microorganisms, is described in detail in PCT Publication No. WO 2004/101757. Palmitate is the precursor of longer-chain saturated and unsaturated fatty acid derivates, which are formed through the action of elongases and desaturases (FIG. 1).

TAGs (the primary storage unit for fatty acids) are formed by a series of reactions that involve: 1.) the esterification of one molecule of acyl-CoA to glycerol-3-phosphate via an acyltransferase to produce lysophosphatidic acid; 2.) the esterification of a second molecule of acyl-CoA via an acyltransferase to yield 1,2-diacylglycerol phosphate (commonly identified as phosphatidic acid); 3.) removal of a phosphate by phosphatidic acid phosphatase to yield 1,2-diacylglycerol (DAG); and 4.) the addition of a third fatty acid by the action of an acyltransferase to form TAG. A wide spectrum of fatty acids can be incorporated into TAGs, including saturated and unsaturated fatty acids and short-chain and long-chain fatty acids.

Biosynthesis of Omega Fatty Acids

The metabolic process wherein oleic acid is converted to ω-3/ω-6 fatty acids involves elongation of the carbon chain through the addition of carbon atoms and desaturation of the molecule through the addition of double bonds. This requires a series of special desaturation and elongation enzymes present in the endoplasmic reticulim membrane. However, as seen in FIG. 1 and as described below, there are often multiple alternate pathways for production of a specific ω-3/ω-6 fatty acid.

Specifically, all pathways require the initial conversion of oleic acid to LA, the first of the ω-6 fatty acids, by a Δ12 desaturase. Then, using the “Δ6 desaturase/Δ6 elongase pathway”, ω-6 fatty acids are formed as follows: (1) LA is converted to GLA by a Δ6 desaturase; (2) GLA is converted to DGLA by a C_(18/20) elongase; and (3) DGLA is converted to ARA by a Δ5 desaturase. Alternatively, the “Δ6 desaturase/Δ6 elongase pathway” can be utilized for formation of ω-3 fatty acids as follows: (1) LA is converted to ALA, the first of the ω-3 fatty acids, by a Δ15 desaturase; (2) ALA is converted to STA by a Δ6 desaturase; (3) STA is converted to ETA by a C_(18/20) elongase; (4) ETA is converted to EPA by a Δ5 desaturase; (5) EPA is converted to DPA by a C_(20/22) elongase; and, (6) DPA is converted to DHA by a Δ4 desaturase. Optionally, ω-6 fatty acids may be converted to ω-3 fatty acids; for example, ETA and EPA are produced from DGLA and ARA, respectively, by Δ17 desaturase activity.

Alternate pathways for the biosynthesis of ω-3/ω-6 fatty acids utilize a Δ9 elongase and Δ8 desaturase. More specifically, LA and ALA may be converted to EDA and ETrA, respectively, by a Δ9 elongase; then, a Δ8 desaturase converts EDA to DGLA and/or ETrA to ETA.

It is contemplated that the particular functionalities required to be expressed in a specific host organism for production of ω-3/ω-6 fatty acids will depend on the host cell (and its native PUFA profile and/or desaturase/elongase profile), the availability of substrate, and the desired end product(s). One skilled in the art will be able to identify various candidate genes encoding each of the enzymes desired for ω-3/ω-6 fatty acid biosynthesis. Useful desaturase and elongase sequences may be derived from any source, e.g., isolated from a natural source (from bacteria, algae, fungi, oomycetes, plants, animals, etc.), produced via a semi-synthetic route or synthesized de novo. Although the particular source of the desaturase and elongase genes introduced into the host is not critical, considerations for choosing a specific polypeptide having desaturase or elongase activity include: 1.) the substrate specificity of the polypeptide; 2.) whether the polypeptide or a component thereof is a rate-limiting enzyme; 3.) whether the desaturase or elongase is essential for synthesis of a desired PUFA; and/or, 4.) co-factors required by the polypeptide. The expressed polypeptide preferably has parameters compatible with the biochemical environment of its location in the host cell (see PCT Publication No. WO 2004/101757 for additional details).

In additional embodiments, it will also be useful to consider the conversion efficiency of each particular desaturase and/or elongase. More specifically, since each enzyme rarely functions with 100% efficiency to convert substrate to product, the final lipid profile of un-purified oils produced in a host cell will typically be a mixture of various PUFAs consisting of the desired ω-3/ω-6 fatty acid, as well as various upstream intermediary PUFAs. Thus, consideration of each enzyme's conversion efficiency is also a variable to consider when optimizing biosynthesis of a desired fatty acid.

With each of the considerations above in mind, candidate genes having the appropriate desaturase and elongase activities (e.g., Δ6 desaturases, C_(18/20) elongases, Δ5 desaturases, Δ17 desaturases, Δ15 desaturases, Δ9 desaturases, Δ12 desaturases, C_(14/16) elongases, C_(16/18) elongases, Δ9 elongases, Δ8 desaturases, Δ4 desaturases and C_(20/22) elongases) can be identified according to publicly available literature (e.g., GenBank), the patent literature, and experimental analysis of organisms having the ability to produce PUFAs. These genes will be suitable for introduction into a specific host organism, to enable or enhance the organism's synthesis of PUFAs.

Identification of Δ17 Desaturases

Sequences of the Δ17 desaturases of the invention, isolated from both Phytophthora sojae and P. ramorum and described herein, were first reported by the U.S. Department of Energy's Joint Genome Institute (JGI; Walnut Creek, Calif.). However, it should be noted that although the sequences were known, their function as a Δ17 desaturase was not elucidated before the findings described herein. Applicants are the first to recognize the specific Δ17 desaturase activity of these polypeptides.

In pursuing the correct activity for these sequences a P. infestans desaturase (“PiD17”; SEQ ID NO:9) was used as a query in BLAST searches against the JGI databases. This lead to the identification of two PiD17 homologs; specifically, the P. sojae homolog, designated herein as “PsD17” (SEQ ID NOs:1 and 2), and the P. ramorum homolog, designated herein as “PrD17” (SEQ ID NOs:5 and 6).

It was desirable to modify the wildtype PsD17 and PrD17 desaturases to optimize the expression of each enzyme for use in an alternate host though codon optimization. Use of host-preferred codons can substantially enhance the expression of the foreign gene encoding the polypeptide. In general, host-preferred codons can be determined within a particular host species of interest by examining codon usage in proteins (preferably those expressed in the largest amount) and determining which codons are used with highest frequency. Subsequently, the coding sequence for a polypeptide of interest having e.g., desaturase activity can be synthesized in whole or in part using the codons preferred in the host species.

The wildtype PsD17 and PrD17 desaturases were codon-optimized for expression in Yarrowia lipolytica, as shown below in Table 3.

TABLE 3 Codon-Optimized Δ17 Desaturases For ω-3 Biosynthesis In Y. lipolytica Total Total Change SEQ Bases Codons In GC ID SEQ Modified Modified Content NOs ID In In From WT Of NOs Codon- Codon- to Codon- Wildtype Of Opt. Opt. Codon- Codon- Opt. (WT) Δ17 WT Δ17 Gene Gene Opt.Gene Opt. Δ17 Δ17 PsD17 1, 2 175 of 168 of 65.1% to 54.5% PsD17S 3, 4 (P. sojae) 1092 bp 364 (16.0%) codons (46.2%) PrD17 5, 6 168 of 160 of 64.4% to 54.5% PrD17S 7, 6 (P. ramorum) 1086 bp 362 (15.5%) codons (44.2%)

One skilled in the art would be able to use the teachings herein to create various other codon-optimized Δ17 desaturases suitable for optimal expression in alternate hosts (i.e., other than Yarrowia lipolytica), based on the wildtype PsD17 and PrD17 sequences. Accordingly, the instant invention relates to any codon-optimized Δ17 desaturase that is derived from the wildtype PsD17 and PrD17 (i.e., encoded by SEQ ID NOs:2 and 6, respectively). This includes, but is not limited to, the nucleotide sequences set forth in SEQ ID NOs:3 and 7, which encode synthetic Δ17 desaturase proteins (i.e., PsD17S and PrD17S, as set forth in SEQ ID NOs:4 and 6, respectively) that were codon-optimized for expression in Yarrowia lipolytica.

More specifically, comparison of the PsD17S nucleotide base and deduced amino acid sequences to public databases reveals that the most similar known sequences (i.e., the Phytophthora infestans “ω3 desaturase” of PCT Publication No. WO 2005/083053; see also GenBank Accession No. CAJ30870) are about 91.4% identical to the amino acid sequence of PsD17S (SEQ ID NO:4) reported herein over a length of 361 amino acids using a Clustal W alignment algorithm (MegAlign™, DNASTAR, supra). In contrast, there was 90.9% identity between the amino acid sequences of the P. infestans “ω3 desaturase” and PsD17 (SEQ ID NO:2).

Comparison of the PrD17S nucleotide base and deduced amino acid sequences to public databases reveals that the most similar known sequences (i.e., the P. infestans “ω3 desaturase”, supra) are about 89.5% identical to the amino acid sequence of PrD17S (SEQ ID NO:6) reported herein over a length of 361 amino acids using a Clustal W alignment algorithm (supra).

Within the context of the present invention preferred amino acid fragments are at least about 70%-80% identical to the PsD17S and PrD17S sequences herein, where those sequences that are at least about 85%-90% identical are particularly suitable and those sequences that are at least about 95% identical are most preferred. Preferred PsD17S and PrD17S encoding nucleic acid sequences corresponding to the instant ORFs are those encoding active proteins and which are at least about 70%-80% identical to the nucleic acid sequences of PsD17S and PrD17S reported herein, respectively, where those sequences that are at least 85%-90% identical are particularly suitable and those sequences that are at least about 95% identical are most preferred.

Upon identification of the Oomycete polypeptides described above, the specificity of each codon-optimized fatty acid desaturase was determined by transformation into a suitable host (i.e., Yarrowia lipolytica) and determination of its effect on the fatty acid profile of the host (Examples 5, 8 and 12). As expected, both PsD17S and PrD17S possessed Δ17 desaturase activity, such that the enzymes were capable of catalyzing conversion of ARA to EPA. Specifically, the ARA to EPA conversion efficiency of PsD17S was 40% and 98% (based on determination in two different strains of Y. lipolytica and under different growth conditions), while the ARA to EPA conversion efficiency of PrD17S was 49%. Conversion efficiency was measured according to the following formula: ([product]/[substrate+product])*100, where ‘product’ includes the immediate product and all products in the pathway derived from it.

Unexpectedly, however, PsD17S additionally possessed limited Δ15 desaturase activity (i.e., the LA to ALA conversion efficiency was 15%), while PrD17S did not (i.e., the LA to ALA conversion efficiency was less than 1%). Thus, the Phytophthora sojae desaturases are defined herein as bifunctional Δ17 desaturases, while the Phytophthora ramorum desaturases are defined herein as monofunctional Δ17 desaturases.

Identification and Isolation of Homologs

Any of the instant desaturase sequences (i.e., PsD17, PrD17, PsD17S, PrD17S) or portions thereof may be used to search for Δ17 desaturase homologs in the same or other bacterial, algal, fungal, Oomycete or plant species using sequence analysis software. In general, such computer software matches similar sequences by assigning degrees of homology to various substitutions, deletions, and other modifications.

Alternatively, any of the instant desaturase sequences or portions thereof may also be employed as hybridization reagents for the identification of Δ17 homologs. The basic components of a nucleic acid hybridization test include a probe, a sample suspected of containing the gene or gene fragment of interest and a specific hybridization method. Probes of the present invention are typically single-stranded nucleic acid sequences that are complementary to the nucleic acid sequences to be detected. Probes are “hybridizable” to the nucleic acid sequence to be detected. Although the probe length can vary from 5 bases to tens of thousands of bases, typically a probe length of about 15 bases to about 30 bases is suitable. Only part of the probe molecule need be complementary to the nucleic acid sequence to be detected. In addition, the complementarity between the probe and the target sequence need not be perfect. Hybridization does occur between imperfectly complementary molecules with the result that a certain fraction of the bases in the hybridized region are not paired with the proper complementary base.

Hybridization methods are well defined. Typically the probe and sample must be mixed under conditions that will permit nucleic acid hybridization. This involves contacting the probe and sample in the presence of an inorganic or organic salt under the proper concentration and temperature conditions. The probe and sample nucleic acids must be in contact for a long enough time that any possible hybridization between the probe and sample nucleic acid may occur. The concentration of probe or target in the mixture will determine the time necessary for hybridization to occur. The higher the probe or target concentration, the shorter the hybridization incubation time needed. Optionally, a chaotropic agent may be added (e.g., guanidinium chloride, guanidinium thiocyanate, sodium thiocyanate, lithium tetrachloroacetate, sodium perchlorate, rubidium tetrachloroacetate, potassium iodide, cesium trifluoroacetate). If desired, one can add formamide to the hybridization mixture, typically 30-50% (v/v).

Various hybridization solutions can be employed. Typically, these comprise from about 20 to 60% volume, preferably 30%, of a polar organic solvent. A common hybridization solution employs about 30-50% v/v formamide, about 0.15 to 1 M sodium chloride, about 0.05 to 0.1 M buffers (e.g., sodium citrate, Tris-HCl, PIPES or HEPES (pH range about 6-9)), about 0.05 to 0.2% detergent (e.g., sodium dodecylsulfate), or between 0.5-20 mM EDTA, FICOLL (Pharmacia Inc.) (about 300-500 kdal), polyvinylpyrrolidone (about 250-500 kdal), and serum albumin. Also included in the typical hybridization solution will be unlabeled carrier nucleic acids from about 0.1 to 5 mg/mL, fragmented nucleic DNA (e.g., calf thymus or salmon sperm DNA, or yeast RNA), and optionally from about 0.5 to 2% wt/vol glycine. Other additives may also be included, such as volume exclusion agents that include a variety of polar water-soluble or swellable agents (e.g., polyethylene glycol), anionic polymers (e.g., polyacrylate or polymethylacrylate) and anionic saccharidic polymers (e.g., dextran sulfate).

Nucleic acid hybridization is adaptable to a variety of assay formats. One of the most suitable is the sandwich assay format. The sandwich assay is particularly adaptable to hybridization under non-denaturing conditions. A primary component of a sandwich-type assay is a solid support. The solid support has adsorbed to it or covalently coupled to it immobilized nucleic acid probe that is unlabeled and complementary to one portion of the sequence.

In additional embodiments, any of the Δ17 desaturase nucleic acid fragments described herein (or any homologs identified thereof) may be used to isolate genes encoding homologous proteins from the same or other bacterial, algal, fungal, Oomycete or plant species. Isolation of homologous genes using sequence-dependent protocols is well known in the art. Examples of sequence-dependent protocols include, but are not limited to: 1.) methods of nucleic acid hybridization; 2.) methods of DNA and RNA amplification, as exemplified by various uses of nucleic acid amplification technologies [e.g., polymerase chain reaction (PCR), Mullis et al., U.S. Pat. No. 4,683,202; ligase chain reaction (LCR), Tabor, S. et al., Proc. Natl. Acad. Sci. U.S.A., 82:1074 (1985); or strand displacement amplification (SDA), Walker, et al., Proc. Nat. Acad. Sci. U.S.A., 89:392 (1992)]; and 3.) methods of library construction and screening by complementation.

For example, genes encoding similar proteins or polypeptides to the Δ17 desaturases described herein could be isolated directly by using all or a portion of the instant nucleic acid fragments as DNA hybridization probes to screen libraries from any desired yeast, fungus or oomycete using methodology well known to those skilled in the art (wherein those yeast or fungus producing EPA [or derivatives thereof] would be preferred). Specific oligonucleotide probes based upon the instant nucleic acid sequences can be designed and synthesized by methods known in the art (Maniatis, supra). Moreover, the entire sequences can be used directly to synthesize DNA probes by methods known to the skilled artisan (e.g., random primers DNA labeling, nick translation or end-labeling techniques), or RNA probes using available in vitro transcription systems. In addition, specific primers can be designed and used to amplify a part of (or full-length of) the instant sequences. The resulting amplification products can be labeled directly during amplification reactions or labeled after amplification reactions, and used as probes to isolate full-length DNA fragments under conditions of appropriate stringency.

Typically, in PCR-type amplification techniques, the primers have different sequences and are not complementary to each other. Depending on the desired test conditions, the sequences of the primers should be designed to provide for both efficient and faithful replication of the target nucleic acid. Methods of PCR primer design are common and well known in the art (Thein and Wallace, “The use of oligonucleotide as specific hybridization probes in the Diagnosis of Genetic Disorders”, in Human Genetic Diseases: A Practical Approach, K. E. Davis Ed., (1986) pp 33-50, IRL: Herndon, Va.; and Rychlik, W., In Methods in Molecular Biology, White, B. A. Ed., (1993) Vol. 15, pp 31-39, PCR Protocols: Current Methods and Applications. Humania: Totowa, N.J.).

Generally two short segments of the instant sequences may be used in PCR protocols to amplify longer nucleic acid fragments encoding homologous genes from DNA or RNA. PCR may also be performed on a library of cloned nucleic acid fragments wherein the sequence of one primer is derived from the instant nucleic acid fragments, and the sequence of the other primer takes advantage of the presence of the polyadenylic acid tracts to the 3′ end of the mRNA precursor encoding eukaryotic genes.

Alternatively, the second primer sequence may be based upon sequences derived from the cloning vector. For example, the skilled artisan can follow the RACE protocol (Frohman et al., Proc. Natl. Acad. Sci. U.S.A., 85:8998 (1988)) to generate cDNAs by using PCR to amplify copies of the region between a single point in the transcript and the 3′ or 5′ end. Primers oriented in the 3′ and 5′ directions can be designed from the instant sequences. Using commercially available 3′ RACE or 5′ RACE systems (Gibco/BRL, Gaithersburg, Md.), specific 3′ or 5′ cDNA fragments can be isolated (Ohara et al., Proc. Natl. Acad. Sci. U.S.A., 86:5673 (1989); Loh et al., Science, 243:217 (1989)).

In other embodiments, any of the Δ17 desaturase nucleic acid fragments described herein (or any homologs identified thereof) may be used for creation of new and improved fatty acid desaturases. As is well known in the art, in vitro mutagenesis and selection, chemical mutagenesis, “gene shuffling” methods or other means can be employed to obtain mutations of naturally occurring desaturase genes. Alternatively, improved fatty acids may be synthesized by domain swapping, wherein a functional domain from any of the Δ17 desaturase nucleic acid fragments described herein are exchanged with a functional domain in an alternate desaturase gene to thereby result in a novel protein.

Methods for Production of Various ω-3 and/or ω-6 Fatty Acids

It is expected that introduction of chimeric genes encoding the Δ17 desaturases described herein (i.e., PsD17, PrD17, PsD17S, PrD17S or other mutant enzymes, codon-optimized enzymes or homologs thereof), under the control of the appropriate promoters will result in increased production of EPA in the transformed host organism, respectively. As such, the present invention encompasses a method for the direct production of PUFAs comprising exposing a fatty acid substrate (i.e., ARA) to the desaturase enzymes described herein (e.g., PsD17, PrD17, PsD17S, PrD17S), such that the substrate is converted to the desired fatty acid product (i.e., EPA).

More specifically, it is an object of the present invention to provide a method for the production of EPA in a host cell (e.g., oleaginous yeast), wherein the host cell comprises:

-   -   a.) an isolated nucleotide molecule encoding a Δ17 desaturase         polypeptide, selected from the group consisting of:         -   i.) an isolated nucleotide molecule encoding a Δ17             desaturase polypeptide having at least 90.9% identity when             compared to a polypeptide having the amino acid sequence as             set forth in SEQ ID NO:2, based on the Clustal method of             alignment;         -   ii.) an isolated nucleotide molecule encoding a Δ17             desaturase polypeptide having at least 91.4% identity when             compared to a polypeptide having the amino acid sequence as             set forth in SEQ ID NO:4, based on the Clustal method of             alignment;         -   iii.) an isolated nucleotide molecule encoding a Δ17             desaturase polypeptide having at least 89.5% identity when             compared to a polypeptide having an amino acid sequence as             set forth in SEQ ID NO:6, based on the Clustal method of             alignment; and,     -   b) a source of ARA;         wherein the host cell is grown under conditions such that the         Δ17 desaturase gene is expressed and the ARA is converted to         EPA, and wherein the EPA is optionally recovered.

The person of skill in the art will recognize that the broad substrate range of the Δ17 desaturase will allow for the use of the enzyme for the conversion of dihomo-γ-linolenic acid to eicosatetraenoic acid. Accordingly the invention provides a method for the production of eicosatetraenoic acid comprising providing a host cell comprising:

-   -   a.) an isolated nucleotide molecule encoding a Δ17 desaturase         polypeptide, selected from the group consisting of:         -   i.) an isolated nucleotide molecule encoding a Δ17             desaturase polypeptide having at least 90.9% identity when             compared to a polypeptide having the amino acid sequence as             set forth in SEQ ID NO:2, based on the Clustal method of             alignment;         -   ii.) an isolated nucleotide molecule encoding a Δ17             desaturase polypeptide having at least 91.4% identity when             compared to a polypeptide having the amino acid sequence as             set forth in SEQ ID NO:4, based on the Clustal method of             alignment; and,         -   iii.) an isolated nucleotide molecule encoding a Δ17             desaturase polypeptide having at least 89.5% identity when             compared to a polypeptide having an amino acid sequence as             set forth in SEQ ID NO:6, based on the Clustal method of             alignment;     -   b.) a source of dihomo-γ-linolenic acid;     -   c.) growing the host cell comprising a nucleotide molecule of         step (a) under conditions wherein the nucleic acid molecule         encoding the Δ17 desaturase polypeptide is expressed and the         dihomo-γ-linolenic acid is converted to eicosatetraenoic acid;         and,     -   d.) optionally recovering the eicosatetraenoic acid of step (c).

In an alternate embodiment, based on the bifunctionality of the Phytophthora sojae Δ17 desaturases, it is an object of the present invention to provide a method for the production of ALA in a host cell (e.g., oleaginous yeast), wherein the host cell comprises:

-   -   a.) an isolated nucleotide molecule encoding a bifunctional Δ17         desaturase polypeptide, selected from the group consisting of:         -   i.) an isolated nucleotide molecule encoding a Δ17             desaturase polypeptide having at least 90.9% identity when             compared to a polypeptide having the amino acid sequence as             set forth in SEQ ID NO:2, based on the Clustal method of             alignment; and,         -   ii.) an isolated nucleotide molecule encoding a Δ17             desaturase polypeptide having at least 91.4% identity when             compared to a polypeptide having the amino acid sequence as             set forth in SEQ ID NO:4, based on the Clustal method of             alignment;     -   b) a source of LA;         wherein the host cell is grown under conditions such that the         bifunctional Δ17 desaturase gene is expressed and the LA is         converted to ALA, and wherein the ALA is optionally recovered.

Alternatively, each Δ17 desaturase gene and its corresponding enzyme product described herein can be used indirectly for the production of ω-3 fatty acids (see PCT Publication No. WO 2004/101757). Indirect production of ω-3/ω-6 PUFAs occurs wherein the fatty acid substrate is converted indirectly into the desired fatty acid product, via means of an intermediate step(s) or pathway intermediate(s). Thus, it is contemplated that the Δ17 desaturases described herein (e.g., PsD17, PrD17, PsD17S, PrD17S or other mutant enzymes, codon-optimized enzymes or homologs thereof) may be expressed in conjunction with additional genes encoding enzymes of the PUFA biosynthetic pathway (e.g., Δ6 desaturases, C_(18/20) elongases, Δ5 desaturases, Δ15 desaturases, Δ9 desaturases, Δ12 desaturases, C_(14/16) elongases, C_(16/18) elongases, Δ9 elongases, Δ8 desaturases, Δ4 desaturases, C_(20/22) elongases) to result in higher levels of production of longer-chain ω-3 fatty acids (e.g., EPA, DPA and DHA). The particular genes included within a particular expression cassette will depend on the host cell (and its PUFA profile and/or desaturase/elongase profile), the availability of substrate and the desired end product(s).

In alternative embodiments, it may be useful to disrupt a host organism's native Δ17 desaturase, based on the complete sequences described herein, the complement of those complete sequences, substantial portions of those sequences, codon-optimized desaturases derived therefrom and those sequences that are substantially homologous thereto. For example, the targeted disruption of the Δ17 desaturase (and optionally a Δ15 desaturase) in a host organism produces a mutant strain that has diminished ability to synthesize ω-3 fatty acids. This mutant strain could be useful for the production of “pure” ω-6 fatty acids (without co-synthesis of ω-3 fatty acids).

Expression Systems, Cassettes and Vectors

The genes and gene products of the instant sequences described herein may be expressed in heterologous host cells. Expression in recombinant hosts may be useful for the production of various PUFA pathway intermediates, or for the modulation of PUFA pathways already existing in the host for the synthesis of new products heretofore not possible using the host.

Expression systems and expression vectors containing regulatory sequences that direct high level expression of foreign proteins are well known to those skilled in the art. Any of these could be used to construct chimeric genes for production of any of the gene products of the instant sequences. These chimeric genes could then be introduced into appropriate host cells via transformation to provide high-level expression of the encoded enzymes.

Vectors or DNA cassettes useful for the transformation of suitable host cells are well known in the art. The specific choice of sequences present in the construct is dependent upon the desired expression products (supra), the nature of the host cell and the proposed means of separating transformed cells versus non-transformed cells. Typically, however, the vector or cassette contains sequences directing transcription and translation of the relevant gene(s), a selectable marker and sequences allowing autonomous replication or chromosomal integration. Suitable vectors comprise a region 5′ of the gene that controls transcriptional initiation (e.g., a promoter) and a region 3′ of the DNA fragment that controls transcriptional termination (i.e., a terminator). It is most preferred when both control regions are derived from genes from the transformed host cell, although it is to be understood that such control regions need not be derived from the genes native to the specific species chosen as a production host.

Initiation control regions or promoters which are useful to drive expression of the instant ORFs in the desired host cell are numerous and familiar to those skilled in the art. Virtually any promoter capable of directing expression of these genes in the selected host cell is suitable for the present invention. Expression in a host cell can be accomplished in a transient or stable fashion. Transient expression can be accomplished by inducing the activity of a regulatable promoter operably linked to the gene of interest. Stable expression can be achieved by the use of a constitutive promoter operably linked to the gene of interest. As an example, when the host cell is yeast, transcriptional and translational regions functional in yeast cells are provided, particularly from the host species (e.g., see U.S. patent application Ser. No. 11/265,761, corresponding to PCT Publication No. WO2006/052870 for preferred transcriptional initiation regulatory regions for use in Yarrowia lipolytica). Any one of a number of regulatory sequences can be used, depending upon whether constitutive or induced transcription is desired, the efficiency of the promoter in expressing the ORF of interest, the ease of construction and the like.

The termination region can be derived from the 3′ region of the gene from which the initiation region was obtained or from a different gene. A large number of termination regions are known and function satisfactorily in a variety of hosts (when utilized both in the same and different genera and species from where they were derived). The termination region usually is selected more as a matter of convenience rather than because of any particular property. Termination control regions may also be derived from various genes native to the preferred hosts. Optionally, a termination site may be unnecessary; however, it is most preferred if included.

As one of skill in the art is aware, merely inserting a gene into a cloning vector does not ensure that it will be successfully expressed at the level needed. In response to the need for a high expression rate, many specialized expression vectors have been created by manipulating a number of different genetic elements that control aspects of transcription, translation, protein stability, oxygen limitation, and secretion from the host cell. More specifically, some of the molecular features that have been manipulated to control gene expression include: 1.) the nature of the relevant transcriptional promoter and terminator sequences; 2.) the number of copies of the cloned gene and whether the gene is plasmid-borne or integrated into the genome of the host cell; 3.) the final cellular location of the synthesized foreign protein; 4.) the efficiency of translation and correct folding of the protein in the host organism; 5.) the intrinsic stability of the mRNA and protein of the cloned gene within the host cell; and, 6.) the codon usage within the cloned gene, such that its frequency approaches the frequency of preferred codon usage of the host cell. Each of these types of modifications are encompassed in the present invention, as means to further optimize expression of the Δ17 desaturases described herein.

Transformation of Host Cells

Once the DNA encoding a polypeptide suitable for expression in an appropriate host cell has been obtained, it is placed in a plasmid vector capable of autonomous replication in the host cell, or it is directly integrated into the genome of the host cell. Integration of expression cassettes can occur randomly within the host genome or can be targeted through the use of constructs containing regions of homology with the host genome sufficient to target recombination with the host locus. Where constructs are targeted to an endogenous locus, all or some of the transcriptional and translational regulatory regions can be provided by the endogenous locus.

Where two or more genes are expressed from separate replicating vectors, it is desirable that each vector has a different means of selection and should lack homology to the other construct(s) to maintain stable expression and prevent reassortment of elements among constructs. Judicious choice of regulatory regions, selection means and method of propagation of the introduced construct(s) can be experimentally determined so that all introduced genes are expressed at the necessary levels to provide for synthesis of the desired products.

Constructs comprising the gene of interest may be introduced into a host cell by any standard technique. These techniques include transformation (e.g., lithium acetate transformation [Methods in Enzymology, 194:186-187 (1991)]), protoplast fusion, biolistic impact, electroporation, microinjection, or any other method that introduces the gene of interest into the host cell.

For convenience, a host cell that has been manipulated by any method to take up a DNA sequence (e.g., an expression cassette) will be referred to as “transformed” or “recombinant” herein. The transformed host will have at least one copy of the expression construct and may have two or more, depending upon whether the gene is integrated into the genome, amplified, or is present on an extrachromosomal element having multiple copy numbers. The transformed host cell can be identified by various selection techniques, as described in PCT Publication No. WO 2004/101757 and PCT Publication No. WO 2005/003310.

Following transformation, substrates suitable for the instant Δ17 desaturases (and, optionally other PUFA enzymes that are co-expressed within the host cell) may be produced by the host either naturally or transgenically, or they may be provided exogenously.

Metabolic Engineering of ω-3 and/or ω-6 Fatty Acid Biosynthesis

Knowledge of the sequences of the present Δ17 desaturases will be useful for manipulating ω-3 and/or ω-6 fatty acid biosynthesis in various host cells. This may require metabolic engineering directly within the PUFA biosynthetic pathway or additional manipulation of pathways that contribute carbon to the PUFA biosynthetic pathway. Methods useful for up-regulating desirable biochemical pathways and down-regulating undesirable biochemical pathways are well known to those skilled in the art. For example, biochemical pathways competing with the ω-3 and/or ω-6 fatty acid biosynthetic pathways for energy or carbon, or native PUFA biosynthetic pathway enzymes that interfere with production of a particular PUFA end-product, may be eliminated by gene disruption or down-regulated by other means (e.g., antisense mRNA).

Detailed discussion of manipulations within the PUFA biosynthetic pathway as a means to increase ARA, EPA or DHA (and associated techniques thereof) are presented in PCT Publication No. WO 2006/055322 [U.S. Patent Publication No. 2006-0094092-A1], PCT Publication No. WO 2006/052870 [U.S. Patent Publication No. 2006-0115881-A1] and PCT Publication No. WO 2006/052871 [U.S. Patent Publication No. 2006-0110806-A1], respectively, as are desirable manipulations in the TAG biosynthetic pathway and the TAG degradation pathway (and associated techniques thereof).

Preferred Hosts for Recombinant Expression of Δ17 Desaturases

Host cells for expression of the instant genes and nucleic acid fragments may include hosts that grow on a variety of feedstocks, including simple or complex carbohydrates, fatty acids, organic acids, oils and alcohols, and/or hydrocarbons over a wide range of temperature and pH values. Based on the needs of the Applicants' Assignee, the genes described in the instant invention were initially isolated for expression in an oleaginous yeast (and in particular Yarrowia lipolytica); however, it is contemplated that because transcription, translation and the protein biosynthetic apparatus is highly conserved, any bacteria, yeast, algae, oomycete and/or filamentous fungus will be a suitable host for expression of the present nucleic acid fragments.

Preferred hosts are oleaginous organisms, such as oleaginous yeast. These oleaginous organisms are naturally capable of oil synthesis and accumulation, wherein the oil can comprise greater than about 25% of the cellular dry weight, more preferably greater than about 30% of the cellular dry weight, and most preferably greater than about 40% of the cellular dry weight. Genera typically identified as oleaginous yeast include, but are not limited to: Yarrowia, Candida, Rhodotorula, Rhodosporidium, Cryptococcus, Trichosporon and Lipomyces. More specifically, illustrative oil-synthesizing yeast include: Rhodosporidium toruloides, Lipomyces starkeyii, L. lipoferus, Candida revkaufi, C. pulcherrima, C. tropicalis, C. utilis, Trichosporon pullans, T. cutaneum, Rhodotorula glutinus, R. graminis and Yarrowia lipolytica (formerly classified as Candida lipolytica).

Most preferred is the oleaginous yeast Yarrowia lipolytica; and, in a further embodiment, most preferred are the Y. lipolytica strains designated as ATCC #76982, ATCC #20362, ATCC #8862, ATCC #18944 and/or LGAM S(7)1 (Papanikolaou S., and Aggelis G., Bioresour. Technol., 82(1):43-9 (2002)).

Specific teachings applicable for engineering EPA and DHA in Y. lipolytica are provided in U.S. patent application Ser. No. 11/265,761 and Ser. No. 11/264,737, respectively. Detailed means for the synthesis and transformation of expression vectors comprising Δ17 desaturases in oleaginous yeast (i.e., Yarrowia lipolytica) are provided in PCT Publications No. WO 2004/101757 and No. WO 2006/052870. The preferred method of expressing genes in this yeast is by integration of linear DNA into the genome of the host; and, integration into multiple locations within the genome can be particularly useful when high level expression of genes are desired [e.g., in the Ura3 locus (GenBank Accession No. AJ306421), the Leu2 gene locus (GenBank Accession No. AF260230), the Lys5 gene locus (GenBank Accession No. M34929), the Aco2 gene locus (GenBank Accession No. AJ001300), the Pox3 gene locus (Pox3: GenBank Accession No. XP_(—)503244; or, Aco3: GenBank Accession No. AJ001301), the Δ12 desaturase gene locus (PCT Publication No. WO 2004/104167), the Lip1 gene locus (GenBank Accession No. Z50020) and/or the Lip2 gene locus (GenBank Accession No. AJ012632)].

Preferred selection methods for use in Yarrowia lipolytica are resistance to kanamycin, hygromycin and the amino glycoside G418, as well as ability to grow on media lacking uracil, leucine, lysine, tryptophan or histidine. In alternate embodiments, 5-fluoroorotic acid (5-fluorouracil-6-carboxylic acid monohydrate; “5-FOA”) is used for selection of yeast Ura⁻ mutants. The compound is toxic to yeast cells that possess a functioning URA3 gene encoding orotidine 5′-monophosphate decarboxylase (OMP decarboxylase); thus, based on this toxicity, 5-FOA is especially useful for the selection and identification of Ura⁻ mutant yeast strains (Bartel, P. L. and Fields, S., Yeast 2-Hybrid System, Oxford University: New York, v. 7, pp 109-147, 1997).

Other preferred microbial hosts include oleaginous bacteria, algae, Oomycetes and other fungi; and, within this broad group of microbial hosts, of particular interest are microorganisms that synthesize ω-3/ω-6 fatty acids. Thus, for example, transformation of Mortierella alpina (which is commercially used for production of ARA) with any of the present Δ17 desaturase genes under the control of inducible or regulated promoters could yield a transformant organism capable of synthesizing EPA. The method of transformation of M. alpina is described by Mackenzie et al. (Appl. Environ. Microbiol., 66:4655 (2000)). Similarly, methods for transformation of Thraustochytriales microorganisms are disclosed in U.S. Pat. No. 7,001,772.

In alternate preferred embodiments, the present invention provides a variety of plant hosts for transformation with the Δ17 desaturases described herein. Plants so transformed can be monocotyledonous plants or dicotyledonous plants, and preferably they belong to a class of plants identified as oleaginous (e.g., oilseed plants). Examples of preferred oilseed plant hosts include, but are not limited to: soybean (Glycine and Soja sp.), corn (Zea mays), flax (Linum sp.), rapeseed (Brassica sp.), primrose, canola, maize, safflower (Carthamus sp.) and sunflower (Helianthus sp.). Means for overexpression of fatty acid desaturases in oilseed plants (e.g., construction of expression cassettes, transformation, selection, etc.) are described in PCT Publication No. WO 2005/047479.

No matter what particular host is selected for expression of the Δ17 desaturases described herein, multiple transformants must be screened in order to obtain a strain displaying the desired expression level and pattern. Such screening may be accomplished by Southern analysis of DNA blots (Southern, J. Mol. Biol., 98:503 (1975)), Northern analysis of mRNA expression (Kroczek, J. Chromatogr. Biomed. Appl., 618 (1-2):133-145 (1993)), Western and/or Elisa analyses of protein expression, phenotypic analysis or GC analysis of the PUFA products.

Fermentation Processes for Omega Fatty Acid Production

The transformed host cell is grown under conditions that optimize expression of chimeric desaturase genes and produce the greatest and most economical yield of desired PUFAs. In general, media conditions that may be optimized include the type and amount of carbon source, the type and amount of nitrogen source, the carbon-to-nitrogen ratio, the amount of different mineral ions, the oxygen level, growth temperature, pH, length of the biomass production phase, length of the oil accumulation phase and the time and method of cell harvest. Yarrowia lipolytica are generally grown in complex media (e.g., yeast extract-peptone-dextrose broth (YPD)) or a defined minimal media that lacks a component necessary for growth and thereby forces selection of the desired expression cassettes (e.g., Yeast Nitrogen Base (DIFCO Laboratories, Detroit, Mich.)).

Fermentation media in the present invention must contain a suitable carbon source. Suitable carbon sources are taught in PCT Publication No. WO 2004/101757. Although it is contemplated that the source of carbon utilized in the present invention may encompass a wide variety of carbon-containing sources, preferred carbon sources are sugars, glycerol, and/or fatty acids. Most preferred is glucose and/or fatty acids containing between 10-22 carbons.

Nitrogen may be supplied from an inorganic (e.g., (NH₄)₂SO₄) or organic (e.g., urea or glutamate) source. In addition to appropriate carbon and nitrogen sources, the fermentation media must also contain suitable minerals, salts, cofactors, buffers, vitamins and other components known to those skilled in the art suitable for the growth of the oleaginous host and promotion of the enzymatic pathways necessary for PUFA production. Particular attention is given to several metal ions (e.g., Fe⁺², Cu⁺², Mn⁺², Co⁺², Zn⁺², Mg⁺²) that promote synthesis of lipids and PUFAs (Nakahara, T. et al., Ind. Appl. Single Cell Oils, D. J. Kyle and R. Colin, eds. pp 61-97 (1992)).

Preferred growth media in the present invention are common commercially prepared media, such as Yeast Nitrogen Base (DIFCO Laboratories, Detroit, Mich.). Other defined or synthetic growth media may also be used and the appropriate medium for growth of the transformant host cells will be known by one skilled in the art of microbiology or fermentation science. A suitable pH range for the fermentation is typically between about pH 4.0 to pH 8.0, wherein pH 5.5 to pH 7.5 is preferred as the range for the initial growth conditions. The fermentation may be conducted under aerobic or anaerobic conditions, wherein microaerobic conditions are preferred.

Typically, accumulation of high levels of PUFAs in oleaginous yeast cells requires a two-stage process, since the metabolic state must be “balanced” between growth and synthesis/storage of fats. Thus, most preferably, a two-stage fermentation process is necessary for the production of PUFAs in Yarrowia lipolytica. This approach is described in PCT Publication No. WO 2004/101757, as are various suitable fermentation process designs (i.e., batch, fed-batch and continuous) and considerations during growth.

EXAMPLES

The present invention is further defined in the following Examples. It should be understood that these Examples, while indicating preferred embodiments of the invention, are given by way of illustration only. From the above discussion and these Examples, one skilled in the art can ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

General Methods

Standard recombinant DNA and molecular cloning techniques used in the Examples are well known in the art and are described by: 1.) Sambrook, J., Fritsch, E. F. and Maniatis, T. Molecular Cloning: A Laboratory Manual; Cold Spring Harbor Laboratory: Cold Spring Harbor, N.Y. (1989) (Maniatis); 2.) T. J. Silhavy, M. L. Bennan, and L. W. Enquist, Experiments with Gene Fusions; Cold Spring Harbor Laboratory: Cold Spring Harbor, N.Y. (1984); and 3.) Ausubel, F. M. et al., Current Protocols in Molecular Biology, published by Greene Publishing Assoc. and Wiley-Interscience, Hoboken, N.J. (1987).

Materials and methods suitable for the maintenance and growth of microbial cultures are well known in the art. Techniques suitable for use in the following examples may be found as set out in Manual of Methods for General Bacteriology (Phillipp Gerhardt, R. G. E. Murray, Ralph N. Costilow, Eugene W. Nester, Willis A. Wood, Noel R. Krieg and G. Briggs Phillips, Eds), American Society for Microbiology: Washington, D.C. (1994)); or by Thomas D. Brock in Biotechnology: A Textbook of Industrial Microbiology, 2^(nd) ed., Sinauer Associates: Sunderland, Mass. (1989). All reagents, restriction enzymes and materials used for the growth and maintenance of microbial cells were obtained from Aldrich Chemicals (Milwaukee, Wis.), DIFCO Laboratories (Detroit, Mich.), GIBCO/BRL (Gaithersburg, Md.), or Sigma Chemical Company (St. Louis, Mo.), unless otherwise specified. E. coli (XL1-Blue) competent cells were purchased from the Stratagene Company (San Diego, Calif.). E. coli strains were typically grown at 37° C. on Luria Bertani (LB) plates.

General molecular cloning was performed according to standard methods (Sambrook et al., supra). DNA sequence was generated on an ABI Automatic sequencer using dye terminator technology (U.S. Pat. No. 5,366,860; EP 272,007) using a combination of vector and insert-specific primers. Comparisons of genetic sequences were accomplished using DNASTAR software (DNA Star, Inc.).

Unless otherwise specified, BLAST (Basic Local Alignment Search Tool; Altschul, S. F., et al., J. Mol. Biol., 215:403-410 (1993) and Nucleic Acids Res., 25:3389-3402 (1997)) searches were conducted to identity isolated sequences having similarity to sequences contained in the BLAST “nr” database (comprising all non-redundant GenBank CDS translations, sequences derived from the 3-dimensional structure Brookhaven Protein Data Bank, the SWISS-PROT protein sequence database, EMBL and DDBJ databases). Sequences were translated in all reading frames and compared for similarity to all publicly available protein sequences contained in the “nr” database, using the BLASTX algorithm (Gish, W. and States, D. J. Nature Genetics, 3:266-272 (1993)) provided by the National Center for Biotechnology Information (NCBI). The results of BLAST comparisons summarizing the sequence to which a query sequence had the most similarity are reported according to the % identity, % similarity, and Expectation value. “% Identity” is defined as the percentage of amino acids that are identical between the two proteins. “% Similarity” is defined as the percentage of amino acids that are identical or conserved between the two proteins. “Expectation value” estimates the statistical significance of the match, specifying the number of matches, with a given score, that are expected in a search of a database of this size absolutely by chance.

The meaning of abbreviations is as follows: “sec” means second(s), “min” means minute(s), “h” means hour(s), “d” means day(s), “μL” means microliter(s), “mL” means milliliter(s), “L” means liter(s), “μM” means micromolar, “mM” means millimolar, “M” means molar, “mmol” means millimole(s), “μmole” mean micromole(s), “g” means gram(s), “μg” means microgram(s), “ng” means nanogram(s), “U” means unit(s), “bp” means base pair(s) and “kB” means kilobase(s).

Transformation and Cultivation of Yarrowia lipolytica

Yarrowia lipolytica strain ATCC #20362 was purchased from the American Type Culture Collection (Rockville, Md.). Y. lipolytica strains were usually grown at 28° C. on YPD agar (1% yeast extract, 2% bactopeptone, 2% glucose, 2% agar).

Transformation of Y. lipolytica was performed according to the method of Chen, D. C. et al. (Appl. Microbiol. Biotechnol., 48(2):232-235 (1997)), unless otherwise noted. Briefly, Yarrowia was streaked onto a YPD plate and grown at 30° C. for approximately 18 hr. Several large loopfuls of cells were scraped from the plate and resuspended in 1 mL of transformation buffer containing: 2.25 mL of 50% PEG, average MW 3350; 0.125 mL of 2 M Li acetate, pH 6.0; 0.125 mL of 2 M DTT; and 50 μg sheared salmon sperm DNA. Then, approximately 500 ng of linearized plasmid DNA was incubated in 100 μl of resuspended cells, and maintained at 39° C. for 1 hr with vortex mixing at 15 min intervals. The cells were plated onto selection media plates and maintained at 30° C. for 2 to 3 days.

For selection of transformants, minimal medium (“MM”) was generally used; the composition of MM is as follows: 0.17% yeast nitrogen base (DIFCO Laboratories, Detroit, Mich.) without ammonium sulfate or amino acids, 2% glucose, 0.1% proline, pH 6.1). Supplements of leucine, lysine and/or uracil were added as appropriate to a final concentration of 0.01% (thereby producing “MMLe”, “MMLys” and “MMU” selection media, each prepared with 20 g/L agar).

Alternatively, transformants were selected on 5-fluoroorotic acid (“FOA”; also 5-fluorouracil-6-carboxylic acid monohydrate) selection media, comprising: 0.17% yeast nitrogen base (DIFCO Laboratories) without ammonium sulfate or amino acids, 2% glucose, 0.1% proline, 75 mg/L uracil, 75 mg/L uridine, 900 mg/L FOA (Zymo Research Corp., Orange, Calif.) and 20 g/L agar.

High Glucose Media (“HGM”) was prepared as follows: 6.3 g/L KH₂PO₄, 27 g/L K₂HPO₄ and 80 g/L glucose (pH 7.5).

Fatty Acid Analysis of Yarrowia lipolytica

For fatty acid analysis, cells were collected by centrifugation and lipids were extracted as described in Bligh, E. G. & Dyer, W. J. (Can. J. Biochem. Physiol., 37:911-917 (1959)). Fatty acid methyl esters were prepared by transesterification of the lipid extract with sodium methoxide (Roughan, G., and Nishida I. Arch Biochem Biophys., 276(1):38-46 (1990)) and subsequently analyzed with a Hewlett-Packard 6890 GC fitted with a 30-m×0.25 mm (i.d.) HP-INNOWAX (Hewlett-Packard) column. The oven temperature was from 170° C. (25 min hold) to 185° C. at 3.5° C./min.

For direct base transesterification, Yarrowia culture (3 mL) was harvested, washed once in distilled water, and dried under vacuum in a Speed-Vac for 5-10 min. Sodium methoxide (100 μl of 1%) was added to the sample, and then the sample was vortexed and rocked for 20 min. After adding 3 drops of 1 M NaCl and 400 μl hexane, the sample was vortexed and spun. The upper layer was removed and analyzed by GC as described above.

Example 1 Identification of a Phytophthora sojae Gene Encoding Δ17 Desaturase

The U.S. Department of Energy's Joint Genome Institute (“JGI”; Walnut Creek, Calif.) created version 1.0 of the Phytophthora sojae genome (estimated genome size is 95 Mbp). This genomic sequence was generated using a whole genome shotgun strategy and comprises a total of 19,276 gene models.

Using the amino acid sequence of the Δ17 desaturase of Phytophthora infestans (GenBank Accession No. CAJ30870; designated as “PiD17” herein and corresponding to SEQ ID NO:9) as a query sequence, a TBLASTN (BLAST protein versus translated nucleotide) search was conducted against JGI's Phytophthora sojae database (using the default parameters available from JGI). One P. sojae ORF located on scaffold 17:338148-339167 was found to share extensive homology with PiD17 (i.e., 91.8% identity and 95.6% similarity, with an Expectation value of 0). Based on this homology, the P. sojae ORF was tentatively identified as a Δ17 desaturase and was designated as “PsD17”. When the 1092 bp DNA sequence of PsD17 (SEQ ID NO:1) was retrieved from the database, it was found to encode a polypeptide of 363 amino acids in length (SEQ ID NO:2). Amino acid sequence alignment using a ClustalW analysis (MegAlign™ program of DNASTAR software) showed that there was 90.9% identity between PiD17 and PsD17; in contrast, the nucleotide sequences shared only 86.6% identity.

The sequence homology of PsD17 to all publicly available protein sequences contained in the “nr” database (see General Methods) was also determined by conducting protein-protein BLAST searches using PsD17 (SEQ ID NO:2) as the query sequence. Based on this analysis, PsD17 was found to share the most homology with the omega-3 fatty acid desaturase of Saprolegnia diclina (GenBank Accession No. AA20444); specifically, PsD17 had 60% identity and 74% similarity with the amino acid sequence of GenBank Accession No. AA20444 with an Expectation value of 7E-117. Additionally, PsD17 had 39% identity and 57% similarity with the amino acid sequence of the fatty acid desaturase of Anabaena variabilis ATCC #29413 (GenBank Accession No. ABA23809), with an Expectation value of 4E-57.

Example 2 Synthesis of a Codon-Optimized Δ17 Desaturase Gene (“PsD17S”) for Yarrowia lipolytica

The codon usage of the Δ17 desaturase gene of Phytophthora sojae was optimized for expression in Yarrowia lipolytica, in a manner similar to that described in U.S. Pat. No. 7,125,672. Specifically, a codon-optimized Δ17 desaturase gene (designated “PsD17S”, SEQ ID NOs:3 and 4) was designed based on the coding sequence of PsD17 (SEQ ID NOs:1 and 2), according to the Yarrowia codon usage pattern (PCT Publication No. WO 2004/101753), the consensus sequence around the ‘ATG’ translation initiation codon, and the general rules of RNA stability (Guhaniyogi, G. and J. Brewer, Gene, 265(1-2):11-23 (2001)). In addition to modification of the translation initiation site, 175 bp of the 1092 bp coding region were modified (16.0%) and 168 codons were optimized (46.2%). The GC content was reduced from 65.1% within the wild type gene (i.e., PsD17) to 54.5% within the synthetic gene (i.e., PsD17S). A NcoI site and NotI sites were incorporated around the translation initiation codon and after the stop codon of PsD17S (SEQ ID NO:3), respectively. FIG. 2 shows a comparison of the nucleotide sequences of PsD17 and PsD17S. At the amino acid level, PsD17S lacked the third and forth amino acid, as compared with the wild type PsD17; thus, the total length of PsD17S is 361 amino acids (SEQ ID NO:4). The designed PsD17S gene was synthesized by GenScript Corporation (Piscataway, N.J.) and cloned into pUC57 (GenBank Accession No. Y14837) to generate pPsD17S (SEQ ID NO:10; FIG. 3A).

Example 3 Generation of Construct pFmPsD17S

The present Example describes the construction of plasmid pFmPsD17S comprising a chimeric FBAINm::PsD17S::XPR gene. Plasmid pFmPsD17S (SEQ ID NO:13; FIG. 3D) was constructed by three-way ligation using fragments from plasmids pKUNF1-KEA, pDMW214 and pPsD17S. Plasmid pFmPsD17S was utilized to test functional expression of PsD17S, as described in Example 5, infra.

Plasmid pKUNF1-KEA

pKUNF1-KEA (SEQ ID NO:11; FIG. 3B) comprises a chimeric FBAINm::E1S::Pex20 gene. The “FBAINm” promoter within this chimeric gene (PCT Publication No. WO 05/049805; also identified as “Fba1+intron” in FIG. 3B) refers to a synthetic promoter that is derived from the “FBAIN” promoter, wherein the FBAIN promoter refers to the 5′ upstream untranslated region in front of the ‘ATG’ translation initiation codon of a fructose-bisphosphate aldolase enzyme (E.C. 4.1.2.13) encoded by the fba1 gene and that is necessary for expression, plus a portion of 5′ coding region that has an intron of the fba1 gene. The FBAINm promoter is modified from FBAIN, in that FBAINm has a 52 bp deletion between the ATG translation initiation codon and the intron of the FBAIN promoter (thereby including only 22 amino acids of the N-terminus) and a new translation consensus motif after the intron. Furthermore, while the FBAIN promoter generates a fusion protein when fused with the coding region of a gene to be expressed, the FBAINm promoter does not generate such a fusion protein.

Table 4 summarizes the components of plasmid pKUNF1-KEA.

TABLE 4 Description Of Plasmid pKUNF1-KEA (SEQ ID NO: 11) RE Sites And Nucleotides Within SEQ ID Description Of Fragment And Chimeric NO: 11 Gene Components PmeI/PacI FBAINm::EL1S::Pex20, comprising: (360-2596) FBAINm: Yarrowia lipolytica FBAINm promoter (PCT Publication No. WO 2005/049805) EL1S: codon-optimized elongase 1 gene (PCT Publication No. WO 2004/101753), derived from Mortierella alpina (GenBank Accession No. AX464731) Pex20: Pex20 terminator sequence of Yarrowia Pex20 gene (GenBank Accession No. AF054613) AscI/BsiWI 5′ portion of Yarrowia Ura3 gene (GenBank Accession 3387-2596 No. AJ306421) PacI/SphI 3′ portion of Yarrowia Ura3 gene (GenBank Accession 6617-6095 No. AJ306421) Plasmid pDMW214

pDMW214 (SEQ ID NO:12; FIG. 3C) is a shuttle plasmid that replicates both in E. coli and Yarrowia lipolytica. It contained the following components:

TABLE 5 Description Of Plasmid pDMW214 (SEQ ID NO: 12) RE Sites And Nucleotides Within SEQ ID Description Of Fragment And Chimeric NO: 12 Gene Components 1150-270 ColE1 plasmid origin of replication 2080-1220 Ampicillin-resistance gene (Amp^(R)) 2979-4256 Yarrowia autonomous replication sequence (ARS18; GenBank Accession No. A17608) PmeI/SphI Yarrowia Leu2 gene (GenBank Accession No. 6501-4256 AF260230) 6501-1 FBAIN::GUS::XPR, comprising: FBAIN: Yarrowia lipolytica FBAIN promoter (PCT Publication No. WO 2005/049805) GUS: E. coli gene encoding β-glucuronidase (Jefferson, R. A. Nature, 14; 342: 837-838 (1989)) XPR: ~100 bp of the 3′ region of the Yarrowia Xpr gene (GenBank Accession No. M17741) Final Construction of Plasmid pFmPsD17S

The PmeI/NcoI fragment of plasmid pKUNF1-KEA (FIG. 3B; comprising the FBAINm promoter) and the NcoI/NotI fragment of plasmid pPsD17S (FIG. 3A; comprising the synthetic Δ17 desaturase gene PsD17S) were used directionally to replace the PmeI/Not I fragment of pDMW214 (FIG. 3C). This resulted in generation of pFmPsD17S (SEQ ID NO:13; FIG. 3D), comprising a chimeric FBAINm::PsD17S::XPR gene. Thus, the components of pFmPsD17S were as described in Table 6 below.

TABLE 6 Description Of Plasmid pFmPsD17S (SEQ ID NO: 13) RE Sites And Nucleotides Within SEQ ID Description Of Fragment And Chimeric NO: 13 Gene Components 5551-1305 Yarrowia autonomous replication sequence (ARS18; GenBank Accession No. A17608) 7769-1269 FBAINm::PsD17S::XPR, comprising: FBAINm: Yarrowia lipolytica FBAINm promoter (PCT Publication No. WO 2005/049805) PsD17S: codon-optimized Δ17 desaturase gene (SEQ ID NO: 3), derived from P. sojae XPR: ~100 bp of the 3′ region of the Yarrowia Xpr gene (GenBank Accession No. M17741) 2418-1538 ColE1 plasmid origin of replication 3348-2488 Ampicillin-resistance gene (Amp^(R)) PmeI/SphI Yarrowia Leu2 gene (GenBank Accession No. 7769-5524 AF260230),

Example 4 Generation of Yarrowia lipolytica Strain Y2047 to Produce About 11% ARA of Total Lipids Via the Δ6 Desaturase/Δ6 Elongase Pathway

The present Example describes the construction of strain Y2047, derived from Yarrowia lipolytica ATCC #20362, capable of producing 11% ARA relative to the total lipids via expression of a Δ6 desaturase/Δ6 elongase pathway (FIG. 4A). Y2047 has been deposited under the terms of the Budapest Treaty and bears the ATCC number PTA-7186. Additionally, construction of Y2047 has been described in co-pending U.S. patent application Ser. No. 11/265,761, herein incorporated by reference.

The development of strain Y2047 first required the construction of strain M4 (producing 8% DGLA).

Generation of M4 Strain to Produce About 8% DGLA of Total Lipids

Construct pKUNF12T6E (FIG. 4B; SEQ ID NO:14) was generated to integrate four chimeric genes (comprising a Δ12 desaturase, a Δ6 desaturase and two C_(18/20) elongases) into the Ura3 loci of wild type Yarrowia strain ATCC #20362, to thereby enable production of DGLA. The pKUNF12T6E plasmid contained the following components:

TABLE 7 Description Of Plasmid pKUNF12T6E (SEQ ID NO: 14) RE Sites And Nucleotides Within SEQ ID Description Of Fragment And Chimeric NO: 14 Gene Components AscI/BsiWI 784 bp 5′ portion of Yarrowia Ura3 gene (GenBank (9420-8629) Accession No. AJ306421) SphI/PacI 516 bp 3′ portion of Yarrowia Ura3 gene (GenBank 12128-1 Accession No. AJ306421) SwaI/BsiWI FBAIN::EL1S::Pex20, comprising: (6380-8629) FBAIN: Yarrowia lipolytica FBAIN promoter (PCT Publication No. WO 2005/049805) EL1S: codon-optimized elongase 1 gene (PCT Publication No. WO 2004/101753), derived from Mortierella alpina (GenBank Accession No. AX464731) Pex20: Pex20 terminator sequence from Yarrowia Pex20 gene (GenBank Accession No. AF054613) BglII/SwaI TEF::Δ6S::Lip1, comprising: (4221-6380) TEF: Yarrowia lipolytica TEF promoter (GenBank Accession No. AF054508) Δ6S: codon-optimized Δ6 desaturase gene (PCT Publication No. WO 2004/101753), derived from Mortierella alpina (GenBank Accession No. AF465281) Lip1: Lip1 terminator sequence from Yarrowia Lip1 gene (GenBank Accession No. Z50020) PmeI/ClaI FBA::F.Δ12::Lip2, comprising: (4207-1459) FBA: Yarrowia lipolytica FBA promoter (PCT Publication No. WO 2005/049805) F.Δ12: Fusarium moniliforme Δ12 desaturase gene (PCT Publication No. WO 2005/047485) Lip2: Lip2 terminator sequence from Yarrowia Lip2 gene (GenBank Accession No. AJ012632) ClaI/PacI TEF::EL2S::XPR, comprising: (1459-1) TEF: Yarrowia lipolytica TEF promoter (GenBank Accession No. AF054508) EL2S: codon-optimized elongase gene (SEQ ID NO: 15), derived from Thraustochytrium aureum (U.S. Pat. No. 6,677,145) XPR: ~100 bp of the 3′ region of the Yarrowia Xpr gene (Gen Bank Accession No. M17741)

The pKUNF12T6E plasmid was digested with AscI/SphI, and then used for transformation of wild type Y. lipolytica ATCC #20362 according to the General Methods. The transformant cells were plated onto FOA selection media plates and maintained at 30° C. for 2 to 3 days. The FOA resistant colonies were picked and streaked onto MM and MMU selection plates. The colonies that could grow on MMU plates but not on MM plates were selected as Ura− strains. Single colonies of Ura− strains were then inoculated into liquid MMU at 30° C. and shaken at 250 rpm/min for 2 days. The cells were collected by centrifugation, lipids were extracted, and fatty acid methyl esters were prepared by trans-esterification, and subsequently analyzed with a Hewlett-Packard 6890 GC.

GC analyses showed the presence of DGLA in the transformants containing the 4 chimeric genes of pKUNF12T6E, but not in the wild type Yarrowia control strain. Most of the selected 32 Ura⁻ strains produced about 6% DGLA of total lipids. There were 2 strains (i.e., strains M4 and 13-8) that produced about 8% DGLA of total lipids.

Generation of Y2047 Strain to Produce About 11% ARA of Total Lipids

Construct pDMW271 (FIG. 4C; SEQ ID NO:17) was generated to integrate three Δ5 chimeric genes into the Leu2 gene of Yarrowia strain M4. Plasmid pDMW271 contained the following components, as described in Table 8:

TABLE 8 Description Of Plasmid pDMW271 (SEQ ID NO: 17) RE Sites And Nucleotides Within SEQ ID Description Of Fragment And Chimeric NO: 17 Gene Components AscI/BsiWI 788 bp 5′ portion of Yarrowia Leu2 gene (GenBank (5520-6315) Accession No. AF260230) SphI/PacI 703 bp 3′ portion of Yarrowia Leu2 gene (GenBank 2820-2109 Accession No. AF260230) SwaI/BsiWI FBAIN::MAΔ5::Pex20, comprising: (8960-6315) FBAIN: Yarrowia lipolytica FBAIN promoter (PCT Publication No. WO 2005/049805) MAΔ5: Mortierella alpina Δ5 desaturase gene (GenBank Accession No. AF067654) Pex20: Pex20 terminator sequence of Yarrowia Pex20 gene (GenBank Accession No. AF054613) SwaI/ClaI TEF::MAΔ5::Lip1, comprising: (8960-11055) TEF: Yarrowia lipolytica TEF promoter (GenBank Accession No. AF054508) MAΔ5: Mortierella alpina Δ5 desaturase gene (GenBank Accession No. AF067654) Lip1: Lip1 terminator sequence of Yarrowia Lip1 gene (GenBank Accession No. Z50020) PmeI/ClaI Yarrowia Ura3 gene (GenBank Accession No. (12690-11055) AJ306421) ClaI/PacI TEF::HΔ5S::Pex16, comprising: (1-2109) TEF: Yarrowia lipolytica TEF promoter (GenBank Accession No. AF054508) HΔ5S: codon-optimized Δ5 desaturase gene (SEQ ID NO: 18), derived from Homo sapiens (GenBank Accession No. NP_037534) Pex16: Pex16 terminator sequence of Yarrowia Pex16 gene (GenBank Accession No. U75433)

Plasmid pDMW271 was digested with AscI/SphI, and then used to transform strain M4 according to the General Methods. Following transformation, the cells were plated onto MMLe plates and maintained at 30° C. for 2 to 3 days. The individual colonies grown on MMLe plates were picked and streaked onto MM and MMLe plates. Those colonies that could grow on MMLe plates but not on MM plates were selected as Leu2⁻ strains. Single colonies of Leu2⁻ strains were then inoculated into liquid MMLe media at 30° C. and shaken at 250 rpm/min for 2 days. The cells were collected by centrifugation, lipids were extracted, and fatty acid methyl esters were prepared by trans-esterification, and subsequently analyzed with a Hewlett-Packard 6890 GC.

GC analyses showed the presence of ARA in pDMW271 transformants, but not in the parental M4 strain. Specifically, among the 48 selected Leu2⁻ transformants with pDMW271, there were 35 strains that produced less than 5% ARA of total lipids, 12 strains that produced 6-8% ARA, and 1 strain that produced about 11% ARA of total lipids in the engineered Yarrowia. The strain that produced 11% ARA was named “Y2047”.

Example 5 Expression of the Codon-Optimized Δ17 Desaturase Gene (“PsD17S”) in Yarrowia lipolytica Strain Y2047

Plasmid pFmPsD17S (FIG. 3D; Example 3) was transformed into Yarrowia lipolytica strain Y2047 (Example 4), as described in the General Methods. The transformant cells were plated onto MM selection media plates and maintained at 30° C. for 2 to 3 days. Eight (8) transformants grown on the MM plates were picked and re-streaked onto fresh MM plates. Once grown, these strains were individually inoculated into 3 mL liquid MM at 30° C. and shaken at 250 rpm/min for 2 days. The cells were collected by centrifugation, lipids were extracted, and fatty acid methyl esters were prepared by trans-esterification, and subsequently analyzed with a Hewlett-Packard 6890 GC.

The GC results showed that there were about 3% ARA and 2% EPA of total lipids produced in all 8 transformants. The conversion efficiency whereby PsD17S converted ARA to EPA in these 8 strains was at an average rate of about 40%. The conversion efficiency was measured according to the following formula: ([product]/[substrate+product])*100, where ‘product’ includes the immediate product and all products in the pathway derived from it. Thus, this experimental data demonstrated that the codon-optimized Δ17 desaturase gene (SEQ ID NO:3) derived from P. sojae efficiently desaturated ARA to EPA.

Example 6 Generation of Construct pZP3-P7U

The present Example describes the construction of plasmid pZP3-P7U comprising a chimeric YAT::PsD17S::Lip1 gene, which was designed to integrate into the Pox3 locus (GenBank Accession No. XP_(—)503244) of the Yarrowia genome. Plasmid pZP3-P7U was utilized to test functional expression of PsD17S, as described in Example 8, infra. The components of pZP3-P7U were as described in Table 9 below.

TABLE 9 Description Of Plasmid pZP3-P7U (SEQ ID NO: 20) RE Sites And Nucleotides Within SEQ ID Description Of Fragment And Chimeric NO: 20 Gene Components AscI/BsiWI 770 bp 5′ portion of Yarrowia Pox3 gene (GenBank (4030-4800) Accession No. AJ001301) PacI/SphI 826 bp 3′ portion of Yarrowia Pox3 gene (GenBank (504-1300) Accession No. AJ001301) ClaI/SwaI YAT::PsD17S::Lip1, comprising: 7133-4960 YAT: Yarrowia YAT promoter (PCT Publication No. WO 2006/052754) PsD17S: codon-optimized Δ17 desaturase gene (SEQ ID NO: 3), derived from P. sojae Lip1: Lip1 terminator sequence of Yarrowia Lip1 gene (GenBank Accession No. Z50020) ClaI/EcoRI LoxP::Ura3::LoxP, comprising: 7133-1 LoxP sequence (SEQ ID NO: 21) Yarrowia Ura3 gene (GenBank Accession No. AJ306421) LoxP sequence (SEQ ID NO: 21)

Example 7 Generation of Yarrowia lipolytica Strain Y4070 to Produce About 12% ARA of Total Lipids Via the Δ9 Elongase/Δ8 Desaturase Pathway

The present Example describes the construction of strain Y4070, derived from Yarrowia lipolytica ATCC #20362, capable of producing about 12% ARA relative to the total lipids via expression of a Δ9 elongase/Δ8 desaturase pathway (FIG. 6A). Strain Y4070 was utilized to test the functional expression of PsD17S in Example 8 and PrD17S in Example 12, infra.

The development of strain Y4070 required the construction of strain Y2224 (a FOA resistant mutant from an autonomous mutation of the Ura3 gene of wildtype Yarrowia strain ATCC #20362), strain Y4001 (producing 17% EDA with a Leu− phenotype), strain Y4001U (producing 17% EDA with a Leu− and Ura− phenotype), strain Y4036 (producing 18% DGLA with a Leu− phenotype) and strain Y4036U (producing 18% DGLA with a Leu− and Ura− phenotype).

Generation of Strain Y2224

Strain Y2224 was isolated in the following manner: Yarrowia lipolytica ATCC #20362 cells from a YPD agar plate (1% yeast extract, 2% bactopeptone, 2% glucose, 2% agar) were streaked onto a MM plate (75 mg/L each of uracil and uridine, 6.7 g/L YNB with ammonia sulfate, without amino acid, and 20 g/L glucose) containing 250 mg/L 5-FOA (Zymo Research). Plates were incubated at 28° C. and four of the resulting colonies were patched separately onto MM plates containing 200 mg/mL 5-FOA and MM plates lacking uracil and uridine to confirm uracil Ura3 auxotrophy.

Generation of Strain Y4001 to Produce about 17% EDA of Total Lipids

Strain Y4001 was created via integration of construct pZKLeuN-29E3 (FIG. 6B). This construct, comprising four chimeric genes (i.e., a Δ12 desaturase, a C_(16/18) elongase and two Δ9 elongases), was integrated into the Leu2 loci of strain Y2224 to thereby enable production of EDA.

Construct pZKLeuN-29E3 contained the components shown in Table 10.

TABLE 10 Description of Plasmid pZKLeuN-29E3 (SEQ ID NO: 22) RE Sites And Nucleotides Within SEQ ID Description Of Fragment And Chimeric NO: 22 Gene Components BsiW I/Asc I 788 bp 3′ portion of Yarrowia Leu2 gene (GenBank (7797-7002) Accession No. AF260230) Sph I/Pac I 703 bp 5′ portion of Yarrowia Leu2 gene (GenBank (4302-3591) Accession No. AF260230) Swa I/BsiW I GPD::F.D12::Pex20, comprising: (10500-7797) GPD: Yarrowia lipolytica GPD promoter (PCT Publication No. WO 2005/003310) F.D12: Fusarium moniliforme Δ12 desaturase gene (PCT Publication No. WO 2005/047485) Pex20: Pex20 terminator sequence from Yarrowia Pex20 gene (GenBank Accession No. AF054613) Bgl II/Swa I Exp pro::EgD9E::Lip1, comprising: (12526-10500) Exp pro: Yarrowia lipolytica export protein (EXP1) promoter (PCT Publication No. WO 2006/052870 and U.S. patent application No. 11/265761) EgD9E: codon-optimized Δ9 elongase (SEQ ID NO: 23), derived from Euglena gracilis (“EgD9eS”; U.S. patent applications No. 11/601563 and No. 11/601564) Lip1: Lip1 terminator sequence from Yarrowia Lip1 gene (GenBank Accession No. Z50020) Pme I/Cla I FBAINm::EgD9S::Lip2, comprising: (12544-1) FBAINm: Yarrowia lipolytica FBAINm promoter (PCT Publication No. WO 2005/049805) EgD9S: codon-optimized Δ9 elongase gene (SEQ ID NO: 23), derived from Euglena gracilis (“EgD9eS”; U.S. patent applications No. 11/601563 and No. 11/601564) Lip2: Lip2 terminator sequence from Yarrowia Lip2 gene (GenBank Accession No. AJ012632) Cla I/EcoR I LoxP::Ura3::LoxP, comprising: (1-1736) LoxP sequence (SEQ ID NO: 21) Yarrowia Ura3 gene (GenBank Accession No. AJ306421) LoxP sequence (SEQ ID NO: 21) EcoR I/Pac I NT::ME3S::Pex16, comprising: (1736-3591) NT: Yarrowia lipolytica YAT1 promoter (Patent Publication No. U.S. 2006/0094102-A1) ME3S: codon-optimized C_(16/18) elongase gene (SEQ ID NO: 25), derived from M. alpina (U.S. patent application No. 11/253882 and also PCT Publication No. WO 2006/052870) Pex16: Pex16 terminator sequence of Yarrowia Pex 16 gene (GenBank Accession No. U75433)

Plasmid pZKLeuN-29E3 was digested with Asc I/Sph I, and then used for transformation of Y. lipolytica strain Y2224 (i.e., ATCC #20362 Ura3−) according to the General Methods. The transformant cells were plated onto MMLeu media plates and maintained at 30° C. for 2 to 3 days. The colonies were picked and streaked onto MM and MMLeu selection plates. The colonies that could grow on MMLeu plates but not on MM plates were selected as Leu− strains. Single colonies of Leu− strains were then inoculated into liquid MMLeu at 30° C. and shaken at 250 rpm/min for 2 days. The cells were collected by centrifugation, lipids were extracted, and fatty acid methyl esters were prepared by trans-esterification, and subsequently analyzed with a Hewlett-Packard 6890 GC.

GC analyses showed the presence of EDA in the transformants containing the 4 chimeric genes of pZKLeuN-29E3, but not in the Yarrowia Y2224 control strain. Most of the selected 36 Leu− strains produced about 12 to 16.9% EDA of total lipids. There were 3 strains (i.e., strains #11, #30 and #34) that produced about 17.4%, 17% and 17.5% EDA of total lipids; they were designated as strains Y4001, Y4002 and Y4003, respectively.

Generation of Strain Y4001U (Leu−, Ura−) to Produce about 17% EDA of Total Lipids

Strain Y4001U was created via temporary expression of the Cre recombinase enzyme in plasmid pY116 (FIG. 6C) within strain Y4001 to produce a Leu− and Ura− phenotype. Construct pY116 contained the fo

TABLE 11 Description of Plasmid pY116 (SEQ ID NO: 27) RE Sites And Nucleotides Within SEQ ID Description Of Fragment And NO: 27 Chimeric Gene Components 1328-448 ColE1 plasmid origin of replication 2258-1398 Ampicillin-resistance gene (Amp^(R)) for selection in E. coli 3157-4461 Yarrowia autonomous replication sequence (ARS18; GenBank Accession No. A17608) PacI/SawI Yarrowia Leu2 gene (GenBank Accession No. 6667-4504 AF260230) Swa I/Pme I GPAT::Cre::XPR2, comprising: (6667-218 GPAT: Yarrowia lipolytica GPAT promoter (PCT Publication No. WO 2006/031937) Cre: Enterobacteria phage P1 Cre gene for recombinase protein (GenBank Accession No. X03453) XPR2: ~100 bp of the 3′ region of the Yarrowia Xpr gene (GenBank Accession No. M17741)

Plasmid pY116 was used for transformation of freshly grown Y4001 cells according to the General Methods. The transformant cells were plated onto MMLeu+Ura plates (MMU plus Leucine) containing 280 μg/mL sulfonylurea and maintained at 30° C. for 3 to 4 days. Four colonies were picked, inoculated into 3 mL liquid YPD media at 30° C. and shaken at 250 rpm/min for 1 day. The cultures were diluted to 1:50,000 with liquid MMLeu+Ura media, and 100 μL was plated onto new YPD plates and maintained at 30° C. for 2 days. Colonies were picked and streaked onto MMLeu and MMLeu+Ura selection plates. The colonies that could grow on MMLeu+Ura plates but not on MMLeu plates were selected and analyzed by GC to confirm the presence of C20:2 (EDA). One strain, having a Leu− and Ura− phenotype, produced about 17% EDA of total lipids and was designated as Y4001U.

Generation of Y4036 Strain to Produce about 18% DGLA of Total Lipids

Construct pKO2UF8289 (FIG. 7A; SEQ ID NO:28) was generated to integrate four chimeric genes (comprising a Δ12 desaturase, one Δ9 elongase and two mutant Δ8 desaturases) into the Δ12 loci of strain Y4001U1, to thereby enable production of DGLA. Construct pKO2UF8289 contained the following components:

TABLE 12 Description of Plasmid pKO2UF8289 (SEQ ID NO: 28) RE Sites And Nucleotides Within SEQ ID Description Of Fragment And NO: 28 Chimeric Gene Components AscI/BsiWI 5′ portion of Yarrowia Δ12 desaturase gene (PCT (10304-9567) Publication No. WO 2004/104167) EcoRI/SphI 3′ portion of Yarrowia Δ12 desaturase gene (PCT (13568-13012) Publication No. WO 2004/104167) SwaI/BsiWI FBAINm::EgD8M::Pex20, comprising: (7055-9567) FBAINm: Yarrowia lipolytica FBAINm promoter (PCT Publication No. WO 2005/049805) EgD8M: Synthetic Δ8 desaturase mutant EgD8S-23 (SEQ ID NO: 29), derived from Euglena gracilis (U.S. patent application No. 11/635258) Pex20: Pex20 terminator sequence from Yarrowia Pex20 gene (GenBank Accession No. AF054613) SwaI/PmeI YAT::F.D12::OCT, comprising: (7055-4581) YAT: Yarrowia lipolytica YAT1 promoter (Patent Publication No. US 2006/0094102-A1) F.D12: Fusarium moniliforme Δ12 desaturase gene (PCT Publication No. WO 2005/047485) OCT terminator sequence of Yarrowia OCT gene (GenBank Accession No. X69988) PmeI/PacI EXP::EgD8M::Pex16, comprising: (4581-2124) EXP: Yarrowia lipolytica export protein (EXP1) promoter (PCT Publication No. WO 2006/052870 and U.S. patent application No. 11/265761) EgD8M: Synthetic Δ8 desaturase mutant EgD8S-23 (SEQ ID NO: 29), derived from Euglena gracilis (U.S. patent application No. 11/635258) Pex16: Pex16 terminator of Yarrowia Pex16 gene (GenBank Accession No. U75433) PmeI/ClaI GPAT::EgD9e::Lip2, comprising: (2038-1) GPAT: Yarrowia lipolytica GPAT promoter (PCT Publication No. WO 2006/031937) EgD9e: Euglena gracilis Δ9 elongase gene (SEQ ID NO: 31) (U.S. patent applications No. 11/601563 and 11/601564) Lip2: Lip2 terminator sequence from Yarrowia Lip2 gene (GenBank Accession No. AJ012632) ClaI/EcoRI LoxP::Ura3::LoxP, comprising: (13568-1) LoxP sequence (SEQ ID NO: 21) Yarrowia Ura3 gene (GenBank Accession No. AJ306421) LoxP sequence (SEQ ID NO: 21)

The pKO2UF8289 plasmid was digested with AscI/SphI, and then used for transformation of strain Y4001U1 according to the General Methods. The transformant cells were plated onto MMLeu plates and maintained at 30° C. for 2 to 3 days. The colonies were picked and streaked onto MMLeu selection plates at 30° C. for 2 days. These cells were then inoculated into liquid MMLeu at 30° C. and shaken at 250 rpm/min for 2 days. The cells were collected by centrifugation, lipids were extracted, and fatty acid methyl esters were prepared by trans-esterification, and subsequently analyzed with a Hewlett-Packard 6890 GC.

GC analyses showed the presence of DGLA in the transformants containing the 4 chimeric genes of pKO2UF8289, but not in the parent Y4001U1 strain. Most of the selected 96 strains produced between 7 and 13% DGLA of total lipids. There were 6 strains (i.e., #32, #42, #60, #68, #72 and #94) that produced about 15%, 13.8%, 18.2%, 13.1%, 15.6% and 13.9% DGLA of total lipids. These six strains were designated as Y4034, Y4035, Y4036, Y4037, Y4038 and Y4039, respectively.

Generation of Strain Y4036U (Leu−, Ura3−) to Produce about 18% DGLA of Total Lipids

Construct pY116 (FIG. 6C; SEQ ID NO:27) was utilized to temporarily express a Cre recombinase enzyme in strain Y4036. This released the LoxP sandwiched Ura3 gene from the genome.

Plasmid pY116 was used to transform strain Y4036 according to the General Methods. Following transformation, the cells were plated onto MMLeu+Ura plates (MMU plus Leucine) and maintained at 30° C. for 2 to 3 days. The individual colonies grown on MMLeu+Ura plates were picked, and streaked into YPD liquid media at 30° C. and shaken at 250 rpm/min for 1 day to cure the pY116 plasmid. The grown cultures were streaked on MMLeu+Ura u plates. After two days at 30° C., the individual colonies were re-streaked on MMLeu+Ura, MMU and MMLeu plates. Those colonies that could grow on MMLeu+Ura, but not on MMU or MMLeu plates were selected. One of these strains with Leu− and Ura− phenotypes was designated as Y4036U (Ura−, Leu−).

Generation of Y4070 Strain to Produce about 12% ARA of Total Lipids

Construct pZKSL-555R (FIG. 7B; SEQ ID NO:32) was generated to integrate three Δ5 desaturase genes into the Lys loci of strain Y4036U, to thereby enable production of ARA. The pZKSL-555R plasmid contained the following components:

TABLE 13 Description of Plasmid pZKSL-555R (SEQ ID NO: 32) RE Sites And Nucleotides Within SEQ ID Description Of Fragment And NO: 32 Chimeric Gene Components AscI/BsiWI 720 bp 5′ portion of Yarrowia Lys5 gene (GenBank (3321-2601) Accession No. M34929) PacI/SphI 687 bp 3′ portion of Yarrowia Lys5 gene (GenBank (6716-6029) Accession No. M34929) BglII/BsiWI EXP::EgD5S::Pex20, comprising: (15-2601) EXP: Yarrowia lipolytica export protein (EXP1) promoter (WO 2006/052870 and U.S. patent application No. 11/265761) EgD5S: codon-optimized Δ5 desaturase (SEQ ID NO: 33), derived from Euglena gracilis (U.S. Provisional Application No. 60/801172) Pex20: Pex20 terminator sequence from Yarrowia Pex20 gene (GenBank Accession No. AF054613) ClaI/PmeI YAT::RD5S::OCT, comprising: (11243-1) YAT: Yarrowia lipolytica YAT1 promoter (Patent Publication US 2006/0094102-A1) RD5S: codon-optimized Δ5 desaturase (SEQ ID NO: 35), derived from Peridinium sp. CCMP626 (U.S. Provisional Application No. 60/801119) OCT: OCT terminator sequence of Yarrowia OCT gene (GenBank Accession No. X69988) EcoRI/PacI FBAIN::EgD5WT::Aco, comprising: (9500-6716) FBAIN: Yarrowia lipolytica FBAIN promoter (WO 2005/049805) EgD5WT: Euglena gracilis Δ5 desaturase (SEQ ID NO: 35; U.S. Provisional Application No. 60/801172) with elimination of internal BglII, HindlII and Ncol restriction enzyme sites Aco: Aco terminator of Yarrowia Aco gene (GenBank Accession No. AJ001300), EcoRIClaI Yarrowia Leu2 gene (GenBank Accession No. M37309) (9500-11243)

The pZKSL-555R plasmid was digested with AscI/SphI, and then used for transformation of strain Y4036U according to the General Methods. The transformant cells were plated onto MMLeuLys plates (MMLeu plus Lysine) and maintained at 30° C. for 2 to 3 days. Single colonies were then re-streaked onto MMLeuLys plates, and then inoculated into liquid MMLeuLys at 30° C. and shaken at 250 rpm/min for 2 days. The cells were collected by centrifugation, lipids were extracted, and fatty acid methyl esters were prepared by trans-esterification, and subsequently analyzed with a Hewlett-Packard 6890 GC.

GC analyses showed the presence of ARA in the transformants containing the 3 chimeric genes of pZKSL-555R, but not in the parent Y4036 strain. Most of the selected 96 strains produced ˜10% ARA of total lipids. There were 4 strains (i.e., #57, #58, #69 and #75) that produced about 11.7%, 11.8%, 11.9% and 11.7% ARA of total lipids. These four strains were designated as Y4068, Y4069, Y4070 and Y4071, respectively. Further analyses showed that the three chimeric genes of pZKSL-555R were not integrated into the Lys5 site in the Y4068, Y4069, Y4070 and Y4071 strains. All strains possessed a Lys+ phenotype. The final genotype of strain Y4070 with respect to wildtype Yarrowia lipolytica ATCC #20362 was Ura3−, Leu+, Lys+, GPD::F.D12::Pex20, YAT::F.D12::OCT, YAT::ME3S::Pex16, GPAT:: EgD9e::Lip2, Exp::EgD9eS::Lip1, FBAINm::EgD9eS::Lip2, FBAINm::EgD8M::Pex20, EXP::EgD8M::Pex16, FBAIN::EgD5WT::Aco, EXP::EgD5S::Pex20, YAT::RD5S::OCT.

Example 8 Expression of the Codon-Optimized Δ17 Desaturase Gene (“PsD17S”) in Yarrowia lipolytica Strain Y4070

Plasmid pZP3-P7U (FIG. 5; Example 6) was digested with AscI/SphI, and then used to transform Yarrowia lipolytica strain Y4070 according to the General Methods. Following transformation, the cells were plated onto MM plates and maintained at 30° C. for 2 to 3 days. Twelve (12) transformants grown on the MM plates were picked and re-streaked onto fresh MM plates. Once grown, these strains were individually inoculated into 3 mL liquid MM at 30° C. and shaken at 250 rpm/min for 2 days. The cells were collected by centrifugation, resuspended in high glucose media, and then grow for 5 days at 30° C. and shaken at 250 rpm/min. The cells were collected by centrifugation, lipids were extracted, and fatty acid methyl esters were prepared by trans-esterification, and subsequently analyzed with a Hewlett-Packard 6890 GC.

The GC results are shown below in Table 14. Fatty acids are identified as 16:0, 16:1, 18:0, 18:1 (oleic acid), LA (18:2), ALA, EDA (20:2), DGLA, ARA, ETrA (20:3), ETA and EPA; and the composition of each is presented as a % of the total fatty acids. “Δ17 Activity” was calculated according to the following formula: ([EPA]/[ARA+EPA])*100 and represents percent substrate conversion to EPA. “Δ15 Activity” was calculated according to the following formula: ([ALA]/[LA+ALA])*100 and represents percent substrate conversion to ALA.

TABLE 14 Functional Analysis Of PsD17S In Yarrowia lipolytica Strain Y4070 pZP3-P7U Transformant Δ17 Δ15 in Y4070 16:0 16:1 18:0 18:1 LA ALA EDA DGLA ARA ETrA ETA EPA Activity % Activity % 1 5.0 1.7 5.0 25.1 26.8 4.8 5.7 0.4 0.0 3.9 1.1 11.6 100 15 2 5.2 1.9 5.3 25.1 28.1 5.1 5.5 0.3 0.0 3.9 1.2 11.7 100 15 3 4.8 2.0 6.7 24.4 29.6 3.5 6.2 0.5 0.5 2.5 0.9 11.8 96 10 4 5.0 2.2 5.6 24.5 28.4 5.4 5.2 0.3 0.0 3.6 1.1 11.3 100 16 5 5.1 2.3 5.4 23.3 29.2 5.5 5.1 0.4 0.2 3.5 1.1 11.3 98 16 6 5.8 2.1 5.8 21.1 31.0 4.1 5.8 0.5 0.6 3.1 1.0 11.5 95 12 7 4.9 2.3 5.5 23.7 29.1 5.4 5.3 0.4 0.0 3.6 1.2 11.4 100 16 8 4.9 2.1 5.6 25.0 28.2 5.3 5.2 0.3 0.0 3.6 1.1 11.3 100 16 9 4.9 2.0 5.7 27.0 26.9 4.9 5.6 0.3 0.0 3.9 1.1 11.3 100 15 10  4.6 2.1 5.8 23.8 29.1 5.0 5.6 0.4 0.5 3.4 1.2 11.5 96 15 11  4.9 1.9 5.6 21.5 31.8 4.2 5.6 0.6 1.8 2.7 1.0 11.6 87 12 12  5.0 2.7 4.8 21.1 31.1 6.4 4.8 0.4 0.0 3.7 1.4 11.5 100 17 Ave. 5.0 2.1 5.6 23.8 29.1 5.0 5.5 0.4 0.3 3.5 1.1 11.5 98 15 STD 0.3 0.3 0.5 1.8 1.6 0.8 0.4 0.1 0.5 0.5 0.1 0.2

The GC results demonstrated that there were an average of 0.3% ARA and 11.5% EPA of total lipids produced in 12 transformants. The conversion efficiency whereby PsD17S converted ARA to EPA in these 12 strains was at an average rate of about 98%. The conversion efficiency was measured according to the following formula: ([product]/[substrate+product])*100, where ‘product’ includes the immediate product and all products in the pathway derived from it. Thus, this experimental data demonstrated that the codon-optimized Δ17 desaturase gene (SEQ ID NO:3) derived from P. sojae efficiently desaturated ARA to EPA.

There were also an average of 5% ALA and 29.1% LA of total lipids produced in the 12 transformants. The conversion efficiency whereby PsD17S converted LA to ALA in these 12 strains was at an average rate of about 15%. Additionally, there were EtrA and ETA produced in all 12 strains, suggesting that PsD17 might be able to convert EDA to EtrA and DGLA to ETA; however, it is difficult to calculate the conversion efficiency in these two cases, since the Δ9 elongase in strain Y4070 could convert the ALA to EtrA and the Δ8 desaturase could convert the EtrA to ETA.

It is clear that PsD17 had both Δ17 and Δ15 desaturase activities, thus functioning as a bifunctional desaturase as defined herein.

Example 9 Identification of a Phytophthora ramorum Gene Encoding Δ17 Desaturase

The U.S. Department of Energy's Joint Genome Institute (“JGI”; Walnut Creek, Calif.) created version 1.0 of the Phytophthora ramorum genome (estimated genome size is 65 Mbp). This genomic sequence was generated using a whole genome shotgun strategy and comprises a total of 16,066 gene models.

In a manner similar to that described in Example 1, the amino acid sequence of PiD17 (SEQ ID NO:9) was used as a query sequence to perform a TBLASTN search against JGI's Phytophthora ramorum database (using the default parameters available from JGI).

Two ORFs were found to share extensive homology with PiD17 in the genome sequence of Phytophthora ramorum. Specifically, ORF 80222 shared 89% identity and 94% similarity with SEQ ID NO:9, with an Expectation value of 0, Similarly, ORF48790 shared up to 40% identity and 61% similarity with SEQ ID NO:9, with an Expectation value of 6E-44. Based on these results, ORF 80222 was tentatively identified as a Δ17 desaturase and was designated as “PrD17”.

When the 1086 bp DNA sequence of PrD17 (SEQ ID NO:5) was retrieved from the database, it was found to encode a polypeptide of 361 amino acids in length (SEQ ID NO:6). Amino acid sequence alignment using a ClustalW analysis (MegAlign™ program of DNASTAR software) showed that there was 89.5% identity between PiD17 and PrD17; in contrast, the nucleotide sequences shared only 85.7% identity.

The sequence homology of PrD17 was in turn compared with all publicly available protein sequences contained in the “nr” database (see General Methods) was also determined by conducting protein-protein BLAST searches using PrD17 (SEQ ID NO:6) as the query sequence. The sequence that showed the highest degree of similarity was that of the omega-3 fatty acid desaturase of Saprolegnia diclina (GenBank Accession No. AA20444), sharing 59% identity and 74% similarity, with an Expectation value of E-124. Additionally, PrD17 had 38% identity and 57% similarity with the amino acid sequence of the fatty acid desaturase of Anabaena variabilis ATCC #29413 (GenBank Accession No. ABA23809), with an Expectation value of 6E-61.

Example 10 Synthesis of a Codon-Optimized Δ17 Desaturase Gene (“PrD17S”) for Yarrowia lipolytica

The codon usage of the Δ17 desaturase gene of Phytophthora ramorum was optimized for expression in Yarrowia lipolytica, in a manner similar to that described in Example 2. Specifically, a codon-optimized Δ17 desaturase gene (designated “PrD17S”, SEQ ID NO:7) was designed based on the coding sequence of PrD17 (SEQ ID NOs:5 and 6), according to the Yarrowia codon usage pattern (PCT Publication No. WO 2004/101753), the consensus sequence around the ‘ATG’ translation initiation codon, and the general rules of RNA stability (Guhaniyogi, G. and J. Brewer, Gene, 265(1-2):11-23 (2001)). In addition to modification of the translation initiation site, 168 bp of the 1086 bp coding region were modified (15.5%) and 160 codons were optimized (44.2%). The GC content was reduced from 64.4% within the wild type gene (i.e., PrD17) to 54.5% within the synthetic gene (i.e., PrD17S). A NcoI site and NotI sites were incorporated around the translation initiation codon and after the stop codon of PrD17S (SEQ ID NO:7), respectively. FIG. 8 shows a comparison of the nucleotide sequences of PrD17 and PrD17S, None of the modifications in the codon-optimized gene changed the amino acid sequence of the encoded protein (SEQ ID NO:6). The designed PrD17S gene was synthesized by GenScript Corporation (Piscataway, N.J.) and cloned into pUC57 (GenBank Accession No. Y14837) to generate pPrD17S (FIG. 9A; SEQ ID NO:38).

Example 11 Generation of Construct pZuFPrD17S

The present Example describes the construction of plasmid pZuFPrD17S (FIG. 9B) comprising a chimeric FBAIN::PrD17S::Pex20 gene. Plasmid pZuFPrD17S was utilized to test functional expression of PrD17S, as described in Example 12, infra.

Specifically, plasmid pZuFPrD17S was constructed by using the NcoI/NotI fragment containing the PrD17S coding region of plasmid pPrD17S to replace the NcoI/NotI fragment of pZuF17 (FIG. 9C; SEQ ID NO:39). Thus, the components of pZuFPrD17S were as described in Table 15 below.

TABLE 15 Description Of Plasmid pZuFPrD17S (SEQ ID NO: 40) RE Sites And Nucleotides Within SEQ ID Description Of Fragment And NO: 40 Chimeric Gene Components EcoRI/ClaI Yarrowia autonomous replication sequence (ARS18; 4203-5599 GenBank Accession No. A17608) EcoRI/BsiWI FBAIN::PrD17S::Pex20, comprising: 7152-1407 FBAIN: Yarrowia lipolytica FBAINm promoter (PCT Publication No. WO 2005/049805) PrD17S: codon-optimized Δ17 desaturase gene (SEQ ID NO: 7), derived from P. ramorum Pex20: Pex20 terminator sequence of Yarrowia Pex20 gene (GenBank Accession No. AF054613) 2443-1563 ColE1 plasmid origin of replication 3373-2513 Ampicillin-resistance gene (Amp^(R)) EcoRI/PacI Yarrowia Ura3 gene (GenBank Accession No. 7130-5619 AJ306421)

Example 12 Expression of the Codon-Optimized Δ17 Desaturase Gene PrD17S in Yarrowia lipolytica Strain Y4070

Plasmid pZuFPrD17S (FIG. 9B; Example 11) was transformed into Yarrowia lipolytica strain Y4070 (Example 8), as described in the General Methods. The transformant cells were plated onto MM selection media plates and maintained at 30° C. for 2 to 3 days. Ten (10) transformants grown on MM plates were picked and re-streaked onto fresh MM plates. Once grown, these strains were individually inoculated into 3 mL liquid MM at 30° C. and shaken at 250 rpm/min for 2 days. The cells were collected by centrifugation, resuspended in high glucose media, and then grow for 5 days at 30° C. and shaken at 250 rpm/min. The cells were collected by centrifugation, lipids were extracted, and fatty acid methyl esters were prepared by trans-esterification, and subsequently analyzed with a Hewlett-Packard 6890 GC.

The GC results are shown below in Table 16. Fatty acids are identified as 16:0, 16:1, 18:0, 18:1 (oleic acid), LA (18:2), ALA, EDA (20:2), DGLA, ARA, ETrA (20:3), ETA and EPA; and the composition of each is presented as a % of the total fatty acids. “Δ17 Activity” was calculated according to the following formula: ([EPA]/[ARA+EPA])*100 and represents percent substrate conversion to EPA. “Δ15 Activity” was calculated according to the following formula: ([ALA]/[LA+ALA])*100 and represents percent substrate conversion to ALA.

TABLE 16 Functional Analysis Of PrD17S In Yarrowia lipolytica Strain Y4070 pZUFPrD17S Transformant Δ17 Δ15 in Y4070 16:0 16:1 18:0 18:1 LA ALA EDA DGLA ARA ETrA ETA EPA Activity % Activity % 1 6.2 2.3 5.3 15.3 38.3 0.5 7.5 2.4 7.3 0.6 0.4 7.6 51 1.3 2 6.5 2.3 5.0 14.0 40.2 0.5 7.2 2.5 7.6 0.5 0.4 7.7 50 1.2 3 5.7 2.1 5.5 15.3 38.7 0.5 7.8 2.5 7.5 0.5 0.5 7.4 50 1.3 4 7.8 2.8 3.8 13.8 40.0 0.3 7.4 2.0 7.7 0.4 0.0 6.3 45 0.7 5 5.5 2.0 5.6 15.7 38.0 0.5 8.0 2.5 7.1 0.5 0.5 7.3 51 1.4 6 7.7 3.0 4.1 13.2 41.0 0.3 6.7 1.8 7.7 0.4 0.0 7.2 48 0.7 7 6.1 1.7 4.8 15.3 38.5 0.4 7.7 2.4 7.7 0.5 0.4 7.3 49 1.1 8 6.9 2.2 4.6 15.5 39.7 0.5 7.2 2.4 7.1 0.5 0.4 7.2 50 1.3 9 7.4 2.7 4.1 14.4 41.0 0.3 7.2 2.3 7.5 0.4 0.0 6.9 48 0.7 10  7.1 2.6 4.4 13.7 40.6 0.5 6.7 2.5 7.7 0.5 0.4 7.3 48 1.2 Ave. 6.7 2.4 4.7 13.4 39.6 0.4 7.3 2.3 7.5 0.5 0.3 7.2 49 1.1 STD 0.8 0.4 0.6 4.3 1.1 0.1 0.4 0.2 0.2 0.1 0.2 0.4

Table 16 showed that there were an average of 7.5% ARA and 7.2% EPA of total lipids produced in the 10 transformants. The conversion efficiency whereby PrD17S converted ARA to EPA in these 10 strains was at an average rate of about 49%. The conversion efficiency was measured as described in Example 8. Thus, this experimental data demonstrated that the codon-optimized Δ17 desaturase gene (SEQ ID NO:5) derived from P. ramorum efficiently desaturated ARA to EPA.

There was an average of 0.4% ALA and 39.6% LA (C18:2) of total lipids produced in the 10 transformants. The conversion efficiency whereby PrD17S converted LA to ALA in these 10 strains was at an average rate of about only 1%.

Thus, PrD17S is identified herein as a monofunctional Δ17 desaturase, catalyzing conversion of ARA to EPA. 

1. An isolated nucleic acid molecule selected from the group consisting of: a.) an isolated nucleotide sequence encoding a Δ17 desaturase enzyme as set forth in SEQ ID NO:7; or, b.) an isolated nucleotide sequence that is completely complementary to (a).
 2. A chimeric gene comprising the isolated nucleic acid molecule of claim 1 operably linked to suitable regulatory sequences.
 3. A transformed Yarrowia sp. comprising the isolated nucleic acid molecule of claim
 1. 